Hi everyone! I’m planning to take the subject tests for Literature and Math 2 in October. I took the SAT for the first time this past May and scored a 2100 overall (760 writing, 740 reading, 600 math). Taking this test blind was just for me to gauge what I need to improve on, and I’m definitely going to retake because of my mediocre math score. I didn’t do any prep for that exam, so I’m hoping if I do some practicing and studying, I can improve. Since I’m planning to take the Lit and Math 2 subject tests, I’m going to get some practice books for those two (do you guys recommend Barron’s or Kaplan?).
My question is whether or not practicing from the subject test books will also prepare me for my SAT retake, since I’d rather just practice from two books instead of also an additional one with normal SAT practice. For those of you who have taken the Lit SAT, Math 2 SAT, and the normal SAT, can you guys please tell me if the Lit and Math are basically just the critical reading and math portion of the normal SAT, but harder? I guess to phrase simply: Can studying for these subject tests also be considered studying for the normal SAT, or are the subject tests totally different in nature?
I hope my question made sense, thanks to all who answer! 
PS: This is kind of a question regarding both SAT subject test prep and normal SAT prep, so I’m posting this in both forums.