There are currently four of them:
_Infinity Welcomes Careful Drivers_ \- co-authored by Rob Grant and Doug Naylor
_Better Than Life_ \- Co-authored by Rob Grant and Doug Naylor
_Last Human_ \- by Doug Naylor
_Backwards_ \- by Rob Grant
There are also various script compilations and such, but I've not included them, as it feels like you're only asking about the tie-in novels.