1 It is more likely to be your English Language book. It could be a book written in English, but it would imply that you only had one book in English, and from the context, I would assume that you meant a book to study English. You could also use the word textbook or coursebook to make it absolutely clear.
3 To me, I forgot my maths means that I can't remember what I was taught- I would say maths book.
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