I'll buy anything Terry Pratchett writes without bothering to find out what it's about first
And as for the rest, I no longer buy automatically, *except for the book series that I just need to complete: i.e. GRR Martin (if he ever gets around to publishing the next book), Robert Jordan's WOT, etc.
*I've gotten burned with this one when the children of some authors took a stab at continuing their parent's legacies (i.e. Pern, Dune) - turned out to be terribly written!