I've had the same experience with studying math intensely feeling like it buffs fluid intelligence, and then I hear from many sources about how skill training is domain-specific and doesn't transfer well (1), which would mean that's impossible, and I'm stuck hoping that intensely studying math for several hours a day is something that's fringe enough that it wouldn't be picked up in research.
But, if the fluid-intelligence buff comes from studying consistently and at the edge of your skill level, then going through the two AoPS algebra books, but not getting to be able to solve 100% of their "challenge" problems, is going to do just as much as getting through one of them and getting to the point of being able to solve all the challenge problems. There's no real shortage of math-at-skill-level to study, but not knowing calculus is absolutely crippling; you can't learn physics, chemistry, economics, programming, or really any higher math, because at a certain point, every just kind of assumes you understand calculus.
(1) According to my memory textbook, two things that enhance transfer-of-learning are learning things deeply and learning them concretely in one context or abstractly and then applying it to a wide variety of contexts. Going through math textbooks, proving every theorem before reading the book's proof, and then applying the theorems to a wide range of context seems to fit both bills, so take +1 plausibility. However, this is one of the less well-supported areas, and the book didn't point to any original studies that I could find (2), so grain of salt.
(2) Which isn't to say the research hasn't been done. Just, the "do things in one context or abstractly and then in multiple contexts" was done by a grad student and then published in a book by his adviser, and the "learn things deeply" was presented as a generic experiment without reference to primary literature.
I agree that not learning calculus is absolutely crippling. If the OP has not yet learned calculus, then I would say learning calculus should take precedence. On the other hand, if the OP has already learned calculus, then depending on the circumstances, studying an AoPS algebra book might not be a bad use of his/her time. (Or really just any AoPS book, really; from my experience with AoPS, they tend to write very good math books--or at least very good practice problems.)
Unfortunately, I have very little to say on the topic of increasing fluid intelligence...
Your job, should you choose to accept it, is to comment on this thread explaining the most awesome thing you've done this month. You may be as blatantly proud of yourself as you feel. You may unabashedly consider yourself the coolest freaking person ever because of that awesome thing you're dying to tell everyone about. This is the place to do just that.
Remember, however, that this isn't any kind of progress thread. Nor is it any kind of proposal thread. This thread is solely for people to talk about the awesome things they have done. Not "will do". Not "are working on". Have already done. This is to cultivate an environment of object level productivity rather than meta-productivity methods.
So, what's the coolest thing you've done this month?