I saw this blog that posted chapters of books so the full books (etc. hunger games, twilight...) and was looking to so something similar. Is it legal to post a full book online? If not can you post a few chapters?
It is illegal copyright infringement to post full books online without permission of the copyright holder.
There are books that you can easily get permission to post online. Here is a list of books licensed under the "creative commons" licenses that can be re-posted (usually with attribution, but check the license for each book): http://wiki.creativecommons.org/Books
The books you mention (hunger games, twilight, etc) do not offer free or open licenses. Their publishers would not be happy to have you give their works away for free on the internet. It is not even legal to post chapters from these books without permission.
It is legal to post excerpts from books under the "fair use" provision of copyright for purposes such as "criticism, comment, news reporting, teaching, scholarship, and research". See: http://www.copyright.gov/fls/fl102.html However, just posting sections for people to read would not qualify as a fair use.