I think the Princeton review books are good. I only use mine for the practice tests, but the friends that I share the book with (saving money, yess~~) like the strategies that are suggested. It's not useful for getting from a 700 to a 800, but the strategies and content taught are great for improving <700 scores.
I'm sure there are LOTS of threads on how to study and they're pretty much the same. Practice regularly. Find and focus on problem areas. Practice some more. Oh and make sure you try a test with time constraints at home. There's no point in celebrating an 800 if it takes you an hour.
When I took the SAT, I answered the questions without rushing. At questions where I was uncertain, I put down an answer and then went back to think about it after I was done the rest. The Princeton review study guide has a chart telling you how many questions you should answer depending on what score you're aiming for (so you don't lose lots of points guessing). If you're not aiming for that 2300+, you can leave some blank.
To improve math, you just have to be familiar with the concepts. Study them the way suggested above. Practice. Because you really want to improve that 390. Same for reading. Also try reading more; it'll increase your vocabulary and get you more familiar with identifying tone of voice/inferences/opinion of writer/and all that jazz. Note that by reading, I mean reading good books, like actual literature with deep themes and unfamiliar words, and not simply easy reads.
You have the summer to use, so go make use of it by studying! Good luck~