The amount of background/specific knowledge you need for passage questions varies from one AAMC test to the next, even from one passage to the other. Obviously discrete questions are more memorization based.
I only have the EK book, but after taking some AAMC practice tests (3,4, and 7), it seems like even the EK book offers up way too many specific details. Maybe people who have taken AAMC 8+ can comment.
There are somethings you have to memorize, even for passage based questions (such as which hormones do what, characteristics of symp vs parasymp, etc), but really focus on the relationships between things when learning. I think the EK book does a great job on pointing out important relationships on the sides of pages.