Up Your Score: The Underground Guide to the SAT has been around so long it is no longer "underground." First published in the 1980's, each edition stays fresh by recruiting top-scoring students as co-authors. The 2013-2014 edition is by Larry Burger, Michael Colton, Jaja Liao, Manek Mistry and Paul Rossi.
This guide is intended as a supplement to The Official SAT Study Guide. Up Your Score does not include any practice exercises. Compared to other supplementary guides, this book includes more advice on studying (how to concentrate, how to improve your memory, best time of day to memorize something, etc) stress reduction, and general test-taking strategies. The section on guessing strategies is one of the most interesting of its type that I've read. There is even a section on bubbling technique. Apparently some students take a ridiculously long time to color in the bubbles. Who knew? Most of the tips are of the type you see in every guide, but there are a few new ones here. This guide is more fun than most.
When it comes to content, there isn't much here. One third of the book is devoted to vocabulary. The book doesn't have a whole lot to say on the rest of the critical reading section. There are 600 words in the vocabulary list. The bad news: they are arranged alphabetically instead of by meaning. The good news: after dozen words or so there is a silly alliterative story using those words designed to help you remember them. Another thing this book does well is to distinguish between words you might need to know to answer the vocabulary questions at the beginning and literary terms you might need to know to answer the passage content questions.
The math section begins on such an elementary level that, honestly, if you don't know how to do that then you aren't the sort of person who can learn math from a book. It then proceeds to brush lightly over far too much material without either giving an opportunity to practice or referring you to appropriate problems in The Official Guide.
The writing section is short and tries to cover both the multiple choice content and essay advice.
I picked up a copy at BJ's for under $10, so the price isn't all that bad. However, if you've already read another guide, this one doesn't have enough new stuff to make it worth buying. Check a copy out at your school or local library and then read just the useful bits. If this will be your first guide it's worth the price, although if you find out that you need more intensive help in a particular section, this one won't cut it for you.