First, I like my monster book singular. Not 15 books. Not 3. One should do.
Then I hesitated between setting specific and general. I want general by meta-setting. In other words, I’m not interested in having a monster boon that will do D&D, Star Wars, Aliens, and Shadowrun, even if they all share the same stats and rules. Sticking to D&D, one monster book is enough for me.
For D&D, I want a baseline general book with plenty of different types of monsters. Then, for further expansion, I much prefer setting-specific or thematic collections.
Besides the already mentioned Monster Overhaul, I prefer monster books to be general-audience readers instead of game books. Books on myths, monsters, ghosts, weird creatures, cryptids, etc. I prefer monster stats to be easily done on a page, bookmark, or business card a la Blog of Holding’s Monster Manual on a Business Card.
Just the basic monster book. I have played too long to need much more. I do like the fluff about whare and how things live and cool ways to use them, but keep it a paragraph or two. Most monsters are meant to be killed and do not need much.
Favorite would be the 3-ring-binder. Was not very useful since I never put the pages back after I took them out for the specific adventure. That and most of the things broke and I needed to reinforcement tabs for them. Cool idea though. Would not buy it now though since computers have the monsters and I just cut/paste the stats I need.
I think setting specific like Threats to the Nentir Vale do a great job of providing more than just stats and generic lore. They build an interconected ecosystem of monsters and built in adventure ideas.
General. With indexes similar to the PF1 Bestiary: CR, terrain, role, etc. If you are going to publish more then one, maintain an online master index so one can quickly reference all the monsters and figure out which book has the one you want.