I write according to several different style manuals. I, however, only own the APA manual. For the other styles, I check out a book at the library on citation as needed or I look online at university websites. You can find good basic references for APA here, MLA here, and "Harvard" style here.
Style manuals are generally upwards of $20, so they are not cheap investments. If you are writing with them all the time I recommend them. The reason is because while you will find information about how to cite sources (the most important part of these guides for non-fiction purposes), you will not find grammar and spelling details, which makes you stand out against others. Here are links to APA, MLA, and Chicago (shuddering). I am not all that fond of Chicago, but the press (AP style is a shortened Chicago style guide for the associated press-do not confuse it with APA style) and many other places are.
Harvard style is used internationally. I always thought it was odd because Harvard is such an American school, but I didn't choose to investigate. In the past week, I was hired to do a Harvard style job. Since this is the second most popular style I write in, I decided it was time for me to expand my style guides. I typed in "Harvard style" on Amazon, and it returned this book. Now, the reviews at the time were mostly about the project managers test book (I complained and it looks like they have fixed it), which this is not, so I didn't have much to go on. I ordered it, and happily began writing, using the guidelines online.
The book arrived, and I was slightly disturbed to see it was for Harvard Law School. Hmmm... why was this international woman wanting me to write a research paper on a non-legal topic using Harvard Law School's citation manual? In fact, why do many of my international employers prefer this style? The book is thick, poorly organized, and primarily addresses legal citation. It looked to be a much heartier read that what the APA style manual is, so I gleaned the citation information and realized everything I had down needed to be fixed. Now, I know I was careful when I looked on the website (listed above), so I went back (unfortunately after changing everything) and discovered that the website was different from the book!!
Well, I can't just let something go - after all my job depended on it in this case. I needed to know which was correct. Guess what? It was the website (prompting more frantic citation changes - my least favorite thing to do). The internationally preferred "Harvard" style is actually put out by Australians. Yep, I spent about $40 on a book I will probably never use (unless a lawyer hires me to edit his citations). Anyone want to buy a slightly used legal citation book?