It's help pages state that BooksOnBoard also offer the Mobipocket format, of which the Kindle format is a relative.
This link may be slightly helpful in fixing Mobipocket books for reading on the Kindle. However, I didn't saw any books who are actually available in the Mobipocket format, just ADE, Microsoft Reader and eReader formats.
That makes it hard to read a book on other devices or on your own terms; BooksOnBoard makes heavy use of DRM in it's eBooks. Fortunately, BooksOnBoard also publishes eBooks in the Microsoft Reader (.lit) format, for which there are conversion tools available. You could convert a .lit book to .txt, which can be read on the Kindle directly (and can be read just about anywhere in any size/color/fontfamily you want).
Do read the EULA; it might prohibit you from converting the Ebooks to .txt format, since it loses its DRM protection. If so, then you can't read the books on your Kindle.