That's a bitch! Here's my in-person from 1996, if it's of any help. It does seem a little off to me on first look, but you should search this site for more examples of his signature.
Also here's a site with genuine Clapton signed books:
I agree. This is a very tough signature. If we knew when it was supposed to be signed, it would be much easier. I have attached an autograph that I got in person 1990 and one from 1997 and you can see how he's signature have declined.