development - Text or source (preferably free) for learning developmental biology

I might have to take a graduate level course in developmental biology, of which I have never taken a course in before. The course website lists no recommended texts. Therefore, I assume we will be doing a more independent study, and I would want to have a book or source to better facilitate that study.

Any recommendation, free or not free, is welcome.

Answer

Gilbert's Developmental Biology is an excellent text. It is in the 11^th edition, though the 6^th edition is available free on NCBI bookshelf.