I'm not a dental student yet, but I've been (almost all the way) through grad school and my opinion is that it's never a bad idea to leaf through a book or two. You never know what's going to seep into your head, stay there, and become useful later on.
Of course, you can't know at this point what is going to be useful and what isn't, as Gavin pointed out, but I'd say if you're curious and want to look at some first-year material, go for it!
This may be the last summer you can relax and do nothing, but another way of looking at it is that it's the last summer you can (a) study at a leisurely pace with no pressure, (b) study only what you think is interesting, and (c) stop when you get sick of it!