As a general rule in fictional multimedia universes there is a hierarchy of 'canon'. The original media always takes primacy (so in BT the sourcebooks take precedence, in Star Wars its the films, Star Trek its the TV show, in Mass Effect its the games, ect). More recent canon also overrides older canon. Then they have a secondary source of canon, which is automatically considered canon unless the primary media overrides it (in BT its the novels, so unless a sourcebook conflicts with the information in a novel it's considered canon).
Then there other media which get less and less canon (tv shows, comics, video games, ect). Again the masters of the original media decide the order of precedence. Generally speaking everything that is part of the Battletech license should be considered canon (although perhaps not exactly) unless contradicted by higher canon unless the creators specify otherwise (or its obviously non-canon, alternative endings in video games would be a good example). Even the BT cartoon is canon, they just altered it a bit to make it official in the sourcebooks, basically the cartoon was an 'actual' TV show in the BT universe based on real events.
See its that simple
P.S: my understanding is that the epubs are right bellow the novels in the canon hierarchy so they are essentially canon until a novel or sourcebook alters it.