I would advise skipping the webforms books. I can't say I'm an ASP.NET expert, but from what I've seen the models are completely different.
In particular, a webforms book is likely to include quite a lot of detail about control and page lifecycles, as well as lots of details about the controls themselves - none of which will be relevant in MVC.
There are only three benefits I can think of from reading a good ASP.NET book first:
- It may cover ASP.NET web services, which may be interesting to you
- It may give basics of HTTP and how ASP.NET fits into IIS, much of which will apply to MVC
- An MVC book may draw comparisons between MVC and webforms - and those comparisons won't mean much if you don't know webforms
None of those benefits seem strong enough to justify buying a webforms book though, IMO (where the cost isn't the financial one, but the time involved in reading a book thoroughly).