Why did this novel become the first international best-seller? It faced tough competition within Spain and around Europe. I can offer nine reasons:

1. Luck. The printing press had been invented in the mid-1400s, and sooner or later some book was going to hit it big. Latin theological works dominated early production, but printers needed to find more income, so they were looking for popular works in the vernacular.
2. Literacy. The ability to read had spread into the nobility and the new middle class. Both wanted new leisure activities.
3. Spanish ascendance. At the time, Spain was the leading European power. People who wanted to study its language and culture looked for Spanish literature.
4. A European hero. The Arthurian cycle had spread across all borders. Amadis came from the fictional kingdom of Gaul and his deeds spanned the continent. Though written in Spanish, it wasn't a quintessential Spanish story.
5. Renaissance resonance. Since the story originated in medieval times, it provided a nostalgic retreat from the conflicts and terrors of the Renaissance. On the other hand, its theme of heroism played to a sense of individual possibility at a time when real-life horizons were expanding through exploration and renewed scholarship.
6. Kingly authority. During the late Middle Ages and Renaissance, kings consolidated their power at the expense of the nobility, despite energetic resistance. Amadis, always focused on a royal court, reinforced regal power, so kings initially encouraged its propagation.
7. Fun, fun, fun. The literary and religious authorities who condemned it as frivolous entertainment had a point. Love, adventure, intrigue—this book is a page-turner, following a proven medieval structure of several interwoven plotlines.
8. Soaring prose. It was written to be elegantly declaimed. The dialogue seems stilted now, but at the time it set the standard for fashionable speech—the "Buffy-speak" of its day.
9. Loyal female readers. More on this later.

(...)

Its popularity marked history in amazing ways.

Conquistadors brought it to the New World, "their heads filled with fantastic notions, their courage spurred by noble examples of the great heroes of chivalry," in the words of historian J.H. Elliott. The novels' heroes invariably won riches and noble titles, honors coveted by the conquistadors. Bernal Díaz del Castillo, one of Cortéz;'s soldiers, wrote that the first sight of the Aztec capital Tenochtitlán "left us astonished, and we said that it seemed like the things and enchantments told in a book about Amadis." The names Patagonia and California were taken from chivalric novels.

Holy Roman Emperor Charles V (1519-1556) and Isabel of Portugal had it read to them during their siesta by a lady-in-waiting.

When Charles imprisoned King Francis I of France in Madrid in 1525, his sister Margarita read it to the French monarch. Francis liked Amadis so much that he asked one of his artillery officials, the bibliophile Nicolas Herberay des Essarts, to translate the book. Herberay went on to write eight of the 24 French sequels, plagiarizing some Spanish novels and changing the ending of Rodríguez de Montalvo's version.

Later, portions of the books' letters, speeches, challenges to enemies, and declarations and lamentations of love, as translated by Herberay, were complied into the Trésors des Amadis by Robert le Magnier as models of urbanity and eloquence. The handbook enjoyed 20 editions between 1559 and 1606, and was translated into German and English.

(...)

But the books had critics. This was clear even as far back as the 1300s, when Pero López de Ayala, Chancellor to King Enrique III of Castile, called them a waste of time and "proven lies."

In France in 1588, Michel de Montaigne called them "childish."

Religious authorities denounced the books because, in the words of Papal legate Antonius Possevinus in 1593, "people turned their back on honest reading, the Sacred Scripture, and works of devotion, letting themselves be dazzled by vain sciences and superstitions like astrology."

In 1666, Spanish friar Benito Remigio Noydens, in Moral History of the God Momo, a tract against novels of chivalry, wrote: "They are golden pills that, with a delicious layer of entertainment, flatter the eyes to fill their mouths with bitterness and to poison their souls with venom. I remember having read of a totally dissolute man who, finding himself struck by a young woman and without any hope of conquering her, resolved to get her by trickery and slyness, and, placing in front of her eyes one of these books with an entertaining title, he put in her heart such ideas of love that, following their example, they rotted in her and ruined her honest estate of modesty and her shame."

In Spain, chivalric novels suffered occasional censorship in the early 16th century, as did many books, due to some specific problem with their content. Eventually the entire genre came under attack. In 1555, they were banned in convents and monasteries, and in 1560 the Inquisition began to make its suspects confess to reading them.

The books had changed the nature of reading. In medieval times, literature had been read out loud, but more and more often people were reading these books silently. Rather than being presented in short segments open to discussion, they were devoured in long afternoons that allowed the reader to identify with the characters, with no one to draw attention to the moral failures of those figures.

During the reign of King Philip II (1556-1598), new books in the genre couldn't be printed, and in 1590, two years after the defeat of the Invincible Armada and the malaise that the loss created, existing books could not be republished. Perhaps Phillip was embarrassed by his brief theatrical career, but the case against the books ran deeper than that.

By the time Miguel de Cervantes published Don Quixote in 1605 (thanks to a lenient censor, Antonio de Herrera), novels of chivalry were seen as a threat to Spain, according to Daniel Eisenberg, former editor of the journal of the Cervantes Society of America. They undermined Spanish cultural centrality because the hero was international. They celebrated rebellion against authority. They included and even celebrated sex outside of wedlock. "They represented something like the pornography of their time," Eisenberg says.

(...)

Despite condemnation by husbands, fathers, clergy, Cervantes, and the government, women kept reading them. We know this because denunciations and anecdotes continued throughout the 17th century.

Unlike many of those hectors, I don't believe women read the books for the sex, though the scene in which Amadis and Oriana consummate their love sings with beauty and joy (Chapter XXXV). Amadis of Gaul has another, more important virtue.

Though I couldn't call the novel feminist, it—and the entire genre—is unusually female-friendly. Even today, some novels barely include women at all, but Amadis teems with ladies and damsels on every page, some in distress and some distressing. Often women and girls serve central roles, like queens, sorceresses, and lovers. Others are messengers, witnesses, assistants, or members of the court. Most don't have names—so any one of them could be Mary Sue. Any female reader, whatever her ambition, could imagine herself into the novel, a fantasy world of danger, adventure, and fun.

But the fun couldn't last forever. Fashion, set by men since they controlled the printing presses, had its effect. In the inventories of libraries of noble families, chivalric novels disappeared as if by magic. Amadis of Gaul was not reprinted until the early 19th century, and some Spanish novels of chivalry were not reprinted until the end of the 20th century. Others remain out of print.

In histories about European literature, Amadis rarely gets mentioned. From the beginning, no one took it seriously. Rodríguez de Montalvo admits in his prologue, "I wished to bring together the writings of light things of little substance...rank fiction rather than chronicles."

It became popular, beloved, re-enacted, and extended by other authors. It was a best-seller —the first one in Europe— historically important, and one of the key works in the development of literature as we know it.

But an exuberant, embarrassing fantasy found no place in the literary canon. Only serious works need apply. The story of the greatest knight who ever lived, full of amazing things found outside the natural order, fell into oblivion.