Infinitesimal Calculus Book

Henle and Kleinberg are really good writers. Things are as clear as they can be, and there seems to be a good sense of humor in various places--certainly something that isn't very common in a math text. (For instance, see the examples of pictorial functions on pages 19-23.) I'm not totally sold on the infinitesimal method--the proofs are all really easy, but (perhaps just because I don't have a good intuition for infinitesimals), I don't feel like I internalized many of them, like maybe they're TOO easy. Having said that, the usual epsilon-delta stuff isn't exactly the world's most intuitive thing, either, so maybe they have a point. Anyway, this is a great intro to the subject, and I'll probably be looking further, so that's as good as you can hope for, yes?