BookBoon.com is an interesting site that publishes free textbooks in PDF format. There are several books in the area of Chemical Engineering and Chemistry written by university lecturers, some of whom are from Imperial College. The books are aimed at undergraduates and give an overview of different subjects. There are also videos of accounting lectures. If you are an undergraduate this site is definitely worth investigating.
One word of caution : there are ads on every third page. Also, you do not have to register in order to download a book but you do have to give them your email address to receive their newsletter. You cannot download a book unless you do this. These issues however are negligible considering that these books are available free to you – legally!