Volo's Guide to Monsters, published by Wizards of the Coast in 2016, is a monster book, expanding on the monsters available to DMs. The book's namesake, Volo, is a real character in the Forgotten Realms, and his book is mentioned several times in published adventures:
On a success, a character discerns that Volo is honest but might be stretching the truth about how much he can pay. (Currently low on cash, Volo is awaiting royalty payments from Volo’s Guide to Monsters. To make more coin, he began work on a new book, Volo’s Guide to Spirits and Specters. As it happens, his knowledge of spirits mostly concerns the alcoholic variety, and the writing has not been going well.)
The legendary world traveler is in Port Nyanzaru to deliver signed copies of his new book, Volo’s Guide to Monsters, to the merchant princes (some of whom he knows well, and others he claims to know well), and to promote his latest work. Characters are most likely to encounter Volo in one of the city’s inns or taverns, or in a merchant prince’s villa.
If the characters buy a copy of Volo’s book (50 gp for a durable hardcover edition), they can put it to use. Any time they want to know lore about a particular monster described in the book, give them useful tidbits from Volo’s Guide to Monsters. Do not impart game statistics, since such information would not be available in-world. If this reference is unavailable, use the information in appendix D.
-Tomb of Annihilation
So you can acquire Volo's Guide both in real life, and in game (but no longer on D&D Beyond).
In similar style, many of the other published 5e sourcebook titles are stylized as they would appear within the game world they describe:
- Xanathar's Guide to Everything
- Sword Coast Adventurer's Guide
- Mordenkainen's Tome of Foes
- Mordenkainen's Fiendish Folio Volume 1
- Guildmaster's Guide to Ravnica
- Van Richten's Guide to Ravenloft
- Explorer's Guide to Wildemount
- Wayfinder's Guide to Eberron
Do any of these books have in-universe counterparts like Volo's Guide?