Are you keen to teach English as a foreign language but think your grammar will let you down? Then a good book for your TEFL course would be English Grammar for Dummies.
The book is not written exclusively for TEFL, but is a good starting point for budding teachers who currently have a very poor grasp of grammar or are worried about this aspect of the course. The book starts off explaining very basic concepts and does so in an easy to follow and lighthearted way. There are tons of examples and lots of good advice. As you gain confidence more difficult grammatical concepts are introduced. All aspects of English grammar are covered in a very comprehensive way, from adverbs to adjectives, punctuation to pronouns.
If your grammar is fairly good then I would recommend a more involved grammar book such as English Grammar In Use to start out with.