I think Erica Meltzer also has a book specific to the ACT, too. I think she is pretty universally accepted as the guru of the verbal section. In fact, she used to work on test design for ACT (or maybe it was SAT). There seems to be more diversity of opinion about various math guides. From what I read on Amazon, reviewers either REALLY liked College Panda or they really didn’t, for some reason.
My daughter is using Khan online and PWN the SAT for math, (I bought it after reading the reviews on Amazon) and she thinks it’s fine for learning shortcuts and new ways to approach problems (and this is a girl who doesn’t enjoy math). The PWN’s reviewers said that it’s best for students who have already achieved at least a 600 (a good basic grasp of material).
Hard to say what you can achieve, and how long that would take. So much of that depends on your current level of proficiency and general aptitude. It’s said that math is somewhat easier to improve by sheer effort because so many of the verbal skills (especially reading speed, comprehension, and intuitive grasp of grammar) are created over a lifetime of reading. If you have always been a voracious reader, a little polishing may be all that is needed, and if not, raising the verbal score significantly may be difficult.
The ACT is a very fast-paced test, and many excellent students have trouble finishing it. If you have trouble with speed, you may be more successful with the SAT.