One of the more common ebook formats.
Use either Acrobat Reader or Foxit to view these file.

Common in ebooks, enables search and has good quality.
However, has one major drawback: cannot be opened from a network resource. File must be local on the computer in order to open it.
So I suggest you convert these books to PDF if you want to use them remotely.
To use just install Windows...

Scanned books format, good in size but you can't search the text.
To open these file download and install WinDjView.