Guide to improving grammar

I'm searching for a reference book to help me improve my grammar. I haven't taken an English class since high school and my degrees are in finance and accounting. Now I work with a few people who aren't from the US (Canada, Spain and Australia). I keep hearing them talk about how Americans are the worst at English and it's making me feel guilty. I feel like I really need to step up my command of the language.

I'm looking for one (or maybe two) reference books that I can casually skim through once and keep at home on my desk for reference. I read William Zissner's "On Writing Well," but it's more about writing in general. It's a great book though! I'm looking for something specifically in the grammar field. I also want to learn all of the tenses and everything else I forgot from middle school. If it makes a difference, I'm more of a logical person and I don't really do artsy things. Anything I write will be about accounting, finance, computing or general business topics.

The only book I have on my consideration list right now is the Chicago Manual of Style. Does anyone have recommendations? Thank you!