I have an email attachment containing an ebook. Clicking on attachment can not open it! How do I transfer it itunes so it can play the ebook in the attachment?

First you need an app that will read the book. If it is an ePub or PDF iBooks will wor fine, and it is free. Once you have iBooks (or another app) installed tap and hold on the attachment in the Mail app. You should get an Open In... Option. One for iBooks, And a general one for other compatible apps. Choose one of these to copy the file to that app, where you should be able to read it.

As a side note, books added to iBooks this way will not sync with iTunes; so either save your book somewhere on the computer for safe keeping, or add the book to iTunes on the computer first then sync it back to the iPad in iBooks.

If you have not been using and syncing with iTunes, I recommend the first method.