The Griffiths review basically has; an overview of each section, tips for S1, sample essays and suggested essay structure for S2, and an unofficial "syllabus" for areas to concentrate your studies for S3. It also has some practice, GAMSAT style, questions for all 3 sections. It comes highly recommended, and it's something I'm definitely glad I bought early on in the process to save wasting time studying things that aren't likely to be on the exam.
A C Grayling's The Meaning of Things gets a good rep for S1 and S2, and I'd back what Marathi said on classic literature and add that great books like Catch-22 and Of Mice and Men are on amazon for ~£1, so buy a few and get reading.