Cracking the GRE Mathematics is what I studied. I took it with Almus and she had a great amount of resources, including past lectures, on her website. She's the one who recommended the book to us.
I've got a copy of the study book from the Princeton Review. I went through it with my adviser and a copy of the practice test and it seems to fit well. I'd recommend it. It's been used for more than just the GRE, too. I use it as a reference for courses!
Hey, I'm also taking the Math Subject test in October. My main recommendation is to look at the Princeton Review study book, which you can get for about $10. It has many practice tests with solutions, as well as excellent review problems. Once you figure out what areas you know and what areas you are weak in, grab an old textbook from your shelf and work through lots of practice problems in those areas.
Official ETS Practice Book: https://www.ets.org/gre/subject/about/content/mathematics
UCLA GRE Prep Course Page: https://www.math.ucla.edu/~chparkin/gre.html
Princeton Review: https://www.amazon.com/gp/product/0375429727?ref_=navm_ftr_fullsite
It does, but it's mostly calculus and linear algebra, a fair amount of beginner analysis and algebra, then a tiny bit of further topics. But I think the further topics are a such a small part they're not necessarily worth the study time, especially since you don't really know which ones you'd get. Also "algorithms" is one of the topics that is listed and usually shows up, which sounded a bit intimidating as algorithms can be a tough CS class, but the way it shows up is it will tell you how to perform something simple like the Euclidean algorithm and ask you what the result of step 3 is.
Here's a study guide: https://www.amazon.com/Cracking-GRE-Mathematics-Subject-Test/dp/0375429727
It'll give you a brief review of every topic followed by a short quiz.
Also the ETS site has a description of the topics covered (and how they're weighted): https://www.ets.org/gre/subject/about/content/mathematics
There should be some practice tests on the ets site too.