With the wealth of SAT prep materials out there, it can be tough to find the best resources for SAT study. I've been tutoring for the SAT for over a decade, and these are the materials I've found to be the most helpful.
SAT General Study
For all-around SAT preparation, nothing beats The Official SAT Study Guide, published by the College Board. With ten full practice tests, this book contains plenty of study material for all sections of the test. Because the questions are written by the College Board and, in many cases, have appeared on actual administered SATs, they accurately reflect what students will see on test day. (I've never found a test written by a third-party company that comes close to matching actual SAT questions, and I do not recommend third-party practice tests for study.) Working through the questions in this book is the best, most effective way for any student to prepare for the test.
In addition to the Official Guide, the College Board has published several SAT practice tests online as PDFs. These tests are invaluable for students who need additional practice questions.
SAT Math Prep
For students who need to brush up on basic math concepts, I recommend The CliffsNotes Math Review for Standardized Tests. This book breaks down all the basic math concepts a student will find on the SAT math section into small, easily understood chunks of information and provides plenty of focused practice problems to drill home each idea. Thorough study of this book will help any student fill in gaps in math knowledge and strengthen any weak areas.
For higher scoring students (650+) who need more practice with the most difficult math problems, I often work from the problem solving sections of The Official Guide for GMAT Quantitative Review and The Official Guide for GMAT Review. These books offer questions comparable both in subject matter and style to the questions found on the SAT. Many of the questions could easily be found on an SAT, but the ending difficulty is quite a bit higher, so a student who masters these questions will be extra ready on test day to tackle the math section.
SAT Writing Prep
To review basic grammar and essay writing for the SAT writing section, I recommend CliffsNotes Writing: Grammar, Usage, and Style. It offers a review of the grammar concepts tested on the SAT, a guide to the errors found on the test, and an essay writing section that covers the most important ideas to review, including outlines, introductions, and connecting ideas.
SAT Vocabulary Prep
Many test prep courses and teachers approach vocabulary study by burdening students with long lists of words to memorize. Unfortunately, I've never found this approach to be effective. Instead, I recommend an approach based on learning the meaning of different word roots, prefixes, and suffixes, and I use the book Building Vocabulary for College as a text for study. By breaking down words into different parts and using context clues, students who follow this method will be ready to tackle any unfamiliar words they encounter on the SAT.
For an online guide to word parts, I recommend MemBean, which has an extensive list of word roots, including related words and definitions.