This year, I want to self-study for a couple AP exams (especially some of the sciences like biology) as my school does not offer any courses in AP subjects. Since I’m going to be a junior this year, I also wanted to take some SAT subject tests. If I prepare for the AP Biology exam in May, for example, will this help for taking the biology subject test? Or do the lessons not overlap at all? What about other subjects? Also, what test prep books would you recommend and how have/are/will you studying for your respective SAT subject tests? Sorry for all these questions; personally, I have no experience with subject tests as I’ve only been concentrating on AP exams thus far 
Yes it would.
I didn’t take AP Bio but I have taken AP Chem, AP Calc BC, SATII Chem, SATII Bio E and SATII Math 2.
For Chemistry, there are quite a bit of overlapping.
For math, you’ll need to study a lot more for the AP exam.
I used Barron’s and Kaplan for SAT II Biology and Barron’s for Chem for about 2 months.
Math 2 I did some practice exams a week before.
Good luck!
@sewook123 Thanks for your tips! I will definitely be using these