Commit 4acd0f49e07 has imposed a maximum of 50 pages per book. Even though an in-game player cannot himself create books with more than 50 pages, the client can handle books with many more pages without any issues.

This limitation has broken books functionality on many servers. For instance, speaking as the author of the Advanced Achievements plugin, players that have received over 50 achievements have been reporting that their books (listing one achievement per page) are now trimmed down to 50 pages, which is a very low limit.

Similar discussions were done in SPIGOT-3214 (and related commit [
fecf4b4137c|https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/commits/fecf4b4137c5ce01fb18b697da244a6753b35c2b]).