I'd say the encyclopedia is less rare then the earth staff, but it doesn't mean that it's common or anything. I'd encourage you to learn how to track in hate, and watch for mini pops if you really want it. It's one of those things that you either have to be super lucky or put the time in and grind in out. This link is a mostly complete guide my guild made for doing Magi, but it absolutely works 100% the same for the Broodmaster. If you want a guide for how to check spawn points using Bind sight, PM me I haven't put that out there yet. It was written by Mikhal and it's superb.
https://wiki.project1999.com/Magi_P%27Tasa