Ebooks are books that are converted into electronic format they can be of many different formats like PDF (Portable Document Format), CHM(Compiled Help File), DOC (Word Documents), HTML ...etc
The biggest advantage of using e-books are they can be used at any computer, copying them and distributing them are very easy.Storage is also not a problem too.Better search and organisation is also another great feature of books in digital format.You can download OR buy books from different websites, in some (most) countries it is iilegal to download books that are under Copyright..
You can buy books from sites like amazon.com but if you are interested in downloading them there are also loads of sites that will allow to do so.Here is a small list of sites where you can get almost all ebooks..Happy reading :)
http://www.giuciao.com
http://www.flazx.com
http://www.ebook3000.com
http://www.freebookspot.com
http://gigapedia.org/
http://www.blueportal.org
http://www.phazeddl
http://www.chipollo.info
http://www.avaxhome.ru
http://www.netz.ru
http://www.xpressionsz.com
http://www.2baksa.net/
http://www.maththinking.com/boat/compute…
http://www.megalinker.tk
http://www2.fixdown.com/en/eall_1.htm
http://www.comms.scitech.susx.ac.uk/fft/
http://www.khayma.com/gosay/book.htm
http://www.gayanb.com/
http://www.alieneyes.wordpress.com
http://www.heatonresearch.com/
http://freecomputerbooks.com/
Doing the following google search may also help you :
"index of" pdf "book name"
The best of all is the following search engine which uses Googles technology to find books very easiy, so you don't need to surf through all the site, it is just a search away.. :)
http://google.com/coop/cse?cx=0064018054…
Thank you
Hope that will help you to find the information you are looking for :)