Reddit mentions of 13th Age Bestiary

There are three Bestiary books I highly recommend, but they are a little stat heavy. However they also give tons of plot hooks, inspiration, lore, society information and adventure ideas. They're amazing Bestiaries, even if you don't use them for the systems they are intended for.

• The 13th Age Bestiary and Lions & Tigers & Owlbears: The 13th Age Bestiary 2: These are the two best bestiaries I've ever seen. Every type of creature gets full coverage, who they are, what they want, where they fit into society, the types of trouble they tend to get involved in, plot hooks, story ideas, full on adventure plans, you name it, these books have it. Such phenomenal books.
  
• Pathfinder RPG: Monster Condex: Never thought I'd see myself recommending a Pathfinder (1E) book, but here it is. This is cut from the same mold as the 13th Age bestiaries. The book is fairly large, coming in at about 250 pages, but limits itself to a total of 20 types of creatures. It digs deep into the lore and just general information about these creatures. Adventure ideas and plot hooks abound. Lots of pictures for things in this book as well.

There's just one core rulebook. The bestiary isn't essential, but it's useful.

Fair warning: My biggest complaint with the system is the book is a bit of a hot mess. It's fine if you're familiar with RPGs, and r/13thage will be able to help you with questions, but it was written with the assumption you know what you're doing (which is a huge shame since otherwise it's a great system for starters). For example, it doesn't explain how to deal with a friendly NPC (basically just stat out an enemy) and only mentions what the six core stats (the same ones as Pathfinder and D&D) actually mean in the index, not in the section about stating out characters. Which, as I said, is fine if you're familiar with D&D, but there are some things that could be better explained.