Generally anything published by the four companies which has done BattleTech products is considered canon (FASA, WizKids, FanPro, Catalyst Game Labs (including BattleCorps)).
Secondary sources are generally books that are considered to have a lot of inconsistencies or which contradict other sources we feel are more correct (like the 1st Edition Periphery Sourcebook, Objective Raids). In addition there are other factors like that the written word is considered more correct than artwork.
Also, in general the unwritten rule is that newer sourcebooks is considered the correct one if there is a continuity clash with an older book.
Other sources we do not consider canon are the MechForce publications, BattleTechnology magazines, any computer games (we might give an acknowledgement nod to some of them like place their planet on the map, but their content is generally not canon).
With regards,
Øystein
