One good source of info about FBReader on N800, is
FBReader on Nokia 770 (Maemo 1.1) – Part I and
Part II. This is the 770, rather than a N800, and it is a bit out of date but still mostly relevant.
The very latest versions of FBReader (since 0.8.10) will add MOBI books automatically to your library. To do this the e-books need to be in your e-book "path". The above Part I link says this is: ~/MyDocs/Books:/media/mmc1/Books by default, but this can be changed under Options (Tool Icon) -> General tab (see the 6th image in Part I - note that clicking on an image brings it up full size).
Adding MOBI books automatically will work best if they are in the most recent version of MOBI, rather than the older metadata-poor version. See
PRC to MOBI, where I recommend using mobi2mobi to update old PRC/MOBI files. On a Windows PC, there is also
Mobi2Mobi GUI.