The PocketBook mobile ADE port is new, so perhaps this is already on your roadmap, but pushing the envelop on ADE ePub is an excellent way to get noticed (and get more sales).
Some implementations (Sony, Jinke, Bookeen) already have a way to access user-provided fonts in ePubs, see for example
Fonts and Epub - What works on Sony, Works for Pocket Pro too!. This is a standard feature of mobile ADE, but the fonts need to be in a particular location (i.e. in a particular folder) - and this location varies between devices. Is this already available on the PocketBooks? If not, this should be very easy to add. Note that on its own these fonts are only available in ePubs that have been modified to reference them, but see userStyle.css below.
One implementation (B&N Nook) allows the user to choose between 3 system fonts as the default ePub font. This is better than nothing, but a much better approach is allowing a userStyle.css - since this gives maximum flexibility (and with the addition of user-provided fonts completely trumps selecting between a small number of system fonts). I have a full description of this in
Needed: Customization for ePub, which is for the EZ Reader (Hanlin) but is equally applicable to any implementation of mobile ADE.