Grammar books
Everybody needs a good grammar book.
The Oxford Practice Grammar series (Oxford University Press) and Grammar in Use (Cambridge English) are both good; I personally prefer Oxford Practice Grammar. Make sure you get a book which has both questions and answers (or tests and a key); and make sure you get the right book for your level of English.
Level: Elementary to Pre-Intermediate (CEFR A1-A2)
Oxford Practice Grammar: Basic
(Norman Coe, Mark Harrison, Ken Paterson, ISBN: 0194579786)
Essential Grammar in Use
(Cambridge, Raymond Murphy, ISBN: 9781107480551)
Level: Intermediate to Upper Intermediate (CEFR B1, B2)
Oxford Practice Grammar: Intermediate
(John Eastwood, ISBN: 9780194309134)
English Grammar in Use
(Cambridge, Raymond Murphy, ISBN: 9780521189392)
Level: Upper Intermediate to Advanced (CEFR B2-C1)
Oxford Practice Grammar: Advanced
(George Yule, ISBN: 9780194309165
Advanced Grammar in Use
(Cambridge, Martin Hewings, ISBN: 9781107539303)
