A good book on Fractional Sobolev space. Does someone can give me a good reference for fractional Sobolev spaces ? I looked on the internet but I didn't find any course on this subject. The only reference I found is the book of Adams, but it's much to complicate for beginners. So if someone could recommand me a good introduction course or book, it would be great.

There is a very nice survey paper by several authors available here. It is not a book, but I am sure you will find it valuable.