I also read 'The Practise of English Teaching' by 'Jeremy Harmer', which is basically the same thing. Don't bother reading anything else.
Anyway, the Cambridge book is good for reference, but when you have to prepare a 60 mins class for the following day, you really want something that is going to be a bit more to the point. I used 'usingenglish' for grammar reference, and all CELTA centres have a library. I also used 'Oxford A-Z of Grammar and Punctuation' by 'John Seely'. Very to the point.
Take into account that when you give a class you will only be expected to get one point across to the students. Present perfect, for example. This is where you'll have to be creative. Do you like cutting things out and sticking them together?
AND, brush up on your tenses. Active and passive.
You'll feel great at the end of your last class
Where are you going to do CELTA?