Click E-Books and find the book you are interested in and the format you want. Then click the link. That will take you to the thread where the actual attachment for the book exists. Then click the attachment and your browser should then ask you what you want to do.
Do not right click and save as. That won't work.
Are you in the USA? If so, have a look at one of the two threads on the Sony Reader credit card offer. You might find the Sony Reader more to your liking for ebook reading then lugging around a tablet.