Am I correct in thinking there is a Nascar series of books out there? Probably Silhouette or Harlequin? If there is, are they all by one author or by multiple authors?

Yeah there is, by Harlequin. Different Authors write books for it, I have a few in my TBR but I haven't read them yet.

There's a 'bigger' book series too..Britton I think wrote them. I've read a couple of them but they're not the harlequins.

The thicker single-titles are actually part of Harlequin's HQN line, I think.

Here's the Harlequin page on it.

I think there are other writers who write about racing.  Erin McCarthy has an upcoming single title set in the NASCAR world.