Where do I find these recipe books?

I see there are recipe books with one-shot recipes for upgrading some of the Lore and Legend armor and accessories, for instance, "Frostfang Lore and Legend Armor".

Where do these books drop?

I recall getting one once, last fall. It would've just been a random drop, like the armor is. I was dinking around in places like Oakmyst Forest, etc, at the time, so I don't think it's anywhere specific.

You get a recipe by examining the piece of gear you want to upgrade.

When you kill a mob and it gives you a level 90 piece of gear, just examine the gear and an option window will give you a choice to claim a recipe scroll

Ah wow, had no idea that was how I got that recipe last year...