I recommend at least the post-Nemesis novels which if there's anything TNG-wise you would consider "relaunch" material, that's it. Others have already listed them but they are:
Death in Winter
Greater Than the Sum
Losing the Peace
I see that as the start of where the authors really begin to have the ability to do what they want with these characters and advance the series plotlines without risk of being contradicted by a later movie. Obviously, some other novels before them do introduce some characters and plots that would be refered to later but that's true with any of the novel series.