You should have had 7 hours of (mandatory) ground-school which should have prepared you for the exam.
The exam will be approximately 75% questions about multi-engine theory. There are several good theory books which you can use to study for this, my personal favourite is the Trevor Thom book, although this should really only substitute what you've learnt in the ground school.
The other 25% will be based on the specific type of aircraft which you are training on, so the best reference to study for this part of the exam is the POH.
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