The Books Tab is a special tab that is only available to Wickerbottom. It allows the player to craft Books, which can be used to activate several events at the cost of sanity. Each book has multiple uses.
Fallen Dragoon Eggs sometimes create Lava Pools on the ground. When Lava pools burn out they leave behind Rocks , Ash and Charcoal. Lava Pools can be extinguished directly with Ice, which leaves behind a single Obsidian instead. This allows Wickerbottom to easily farm Rocks, Charcoal and Obsidian.
Even though reading books lowers the player's sanity, one can keep reading the books even at 0 sanity with no restriction on the book's use and effects. It is therefore recommended to grow self-harvesting farms (with Lureplants) or build Tentacle death pits in a single session, reading as many books as possible, and only restoring sanity (with Jerky, Cooked Green Cap, or Cooked Cactus Flesh) afterwards.
Tentacles from On Tentacles generally produces more Tentacle Spots than it consumes. They are also a source of Tentacle Spikes, a great melee weapon. The book can be used to build defenses or traps, as the Tentacles are hostile to most Mobs in the game.
The text "Ia! Ia!" in the description for The End is Nigh! refers to the phrase "Iä! Iä! Cthulhu fhtagn!", a chant from the Cthulhu Mythos of H. P. Lovecraft.
The sanity reduction from reading possibly could suggest that these books contain knowledge which humans were never meant to know, similar to the Necronomicon and Pnakotic Manuscripts from the writings of H.P. Lovecraft.
The Joy of Volcanology is essentially the Volcano Staff, except less accurate.
According to Wilson in Don't Starve Together, Sleepytime Stories is "just 500 pages of telegraph codes". This implies that the book bores those affected to sleep.
Sleepytime Stories and The End is Nigh! were originally named Bedtime Stories and Conflagration respectively.
Wickerbottom's quote for Birds Of The World references the Megascops kennicottii, or western screech owl. Possibly an easter egg to the Screecher Mod.