Montalvo hoped to start an epic series, and he succeeded.
The story of Amadis of Gaul dates back to the 1300s. The oldest version we have was printed in 1508, and is divided into four books, but originally there had been only three. The author, Garcí Rodríguez de Montalvo, explains the reason for the addition in the prologue:
“I corrected these three books of Amadis, such as they could be read, due to poor writers or very corrupt and dissolute scribes, and I translated and emended a fourth book and The Exploits of Esplandian, a sequel, which up until now no one can recall seeing. By great good fortune, it had been discovered in a stone tomb beneath a hermitage near Constantinople and was brought by a Hungarian merchant to eastern Spain in such ancient script and old parchment that it could only be read with much difficulty by those who knew the language.”
Of course, that’s not quite true. The third book ended tragically, which satisfied Medieval tastes, but Montalvo gave it a more optimistic Renaissance outcome, which allowed him to write a fourth book. He also added “doctrinal improvements” to Amadis of Gaul that you may have already noticed, adding a kind of piety more in keeping with Renaissance thought.
The “fifth book,” The Exploits of Esplandian (Las Sergas de Esplandián), opens:
“Here begins the branch from the four books about Amadis, called Las Sergas de Esplandían, which was written in Greek by the hand of the great master Elisabat, who saw and heard of many of his great deeds... and was translated into many languages for the provinces and kingdoms where... having read of the great things of his father, they were eager to see those of his son.”
The book ends inviting other writers to continue the stories, and that came to pass. Las Sergas de Esplandián enjoyed ten edition between 1510 and 1588, and during that century, eight more books were written about Amadis’s family in Spain, and those, too, achieved great success over the decades. (The exception is Book VIII, Lisuarte de Grecia by Juan Díaz, published in 1526, which was overly pious and boring, so it sold badly.) If that were not enough, another 75 novels of chivalry about other knights were published in Spain in the 1500s.
In addition to translations of the Amadis books into several languages, the Amadis family story was extended in Italy, Germany, and France with many additional sequels. In the end, the family tree grew into a sequoia. You can see a pdf of the Amadis clan including the Spanish and Italian branches, published by the Biblioteca Nacional de España, here (scroll down).
Unlike his father, Esplandian fights infidels in war, not individual fellow Christian knights. In much of the book, the son leads a crusade to protect Constantinople from the siege of King Armato of Persia and his Islamic allies. (Constantinople fell in 1453 to the Ottomans, and Sergas was published in 1510, so that was either wishful thinking or alternate history.)
Esplandian also fought for different reasons – not for earthly fame and honor, as he himself explains in Chapter II:
“...if the great things that my father did in this world with so much effort and such a courageous heart and no small danger to his life, so exceedingly well and amid such good men, had been employed in service to the Lord, there could never have been any man equal or comparable to his virtue and valor. But instead he has eagerly sought the things of this mortal world rather than those that shall last forever.... So may it please the Lord on High that, while I resemble my father somewhat, if I exceed his skill, it may be by aiming more to save my soul than to honor my body, and by avoiding everything that may offend Him.”
Montalvo, however, sought worldly fame, “wishing that some shadow of remembrance remain of me,” and shamelessly advertised the sequel to Amadis of Gaul within its “corrected” text. That’s one of the things we can remember him for.