HI Christine, I definitely encourage you to check out my free webinar on May 1 or May 14. Materials needed to prepare (as well as time commitment, etc.) are something I cover.
You should at the very least plan to purchase the IAAP HQ 2018 CAP study guide for the Fall exam. I do not know if they will update it between now and the spring 2020 exam, so if you plan to sit for that test you should probably hold off a bit to find out when the next version will be released.
As for the other books in the body of knowledge, if you plan to join a study group, check with what they require before you buy. If you plan to self-study, plan to pick up a few books. You definitely don't need ALL of them. There is a fair amount of redundancy in that list. I would at least get books in the areas where you aren't as strong or experienced. I think most people can get along with 3-4 books if they are self studying and be sufficiently prepared.
Marie
P.S. past students and colleagues, thank you so much for sharing information on my study groups and webinar. You guys are the BEST and I appreciate you so much!