There are three ways: a) put it in a folder that FBReader is
monitoring for ebooks, b) add it to the library inside FBReader by
selecting the add books icon, and c) right click on the ebook file and
select FBReader to open it. If you do (c) the ebook won't get added
to your library, but your last read location will be remembered.
This works if the ebook is a) DRM-free, and b) in a format that
FBReader can read.
Many ebooks you buy have DRM (encryption), although there are ways to
circumvent the DRM (see
mobileread.com forums for a discussion of DRM)
the DRM-ridden version isn't readable by FBReader.
Three good formats for FBReader (when DRM-free) are ePub, FB2, and
MOBI. Many others are supported, but there are some that are not.