I am at this time familiar with four other books which use infinitesimals in whole or in part to explain the calculus, an approach which was ruled out in the 19th century but has more recently been shown (by Abraham Robinson) to be mathematically justified, and an approach that is closer to the way most scientists USE calculus than the rigorously (epsilon/delta) defined limits of traditional calculus texts. One of those four, by H. J. Keisler (who edited Robinson's papers) is now out of print, though I have been informed that it is available by download on the Internet (of course, printing a book of over 1000 pages will add to your costs!). The other three have been earlier reviewed by me. Henle and Kleinberg's book, "Infinitesimal Calculus," is a slim book, which explains the concepts well, but does not go much into the applications or techniques. I gave it 4 stars, because what it does, it does well, but it doesn't do everything one might want in a calculus text. Bell's book, "A Primer of Infinitesimal Analysis," is harder going but covers more in the way of applications. Neither of the two provides a lot of practice in techniques. Sparks' book, "Calculus Without Limits: Almost," is a very intuitive approach, geared very much to the prospective working scientist, but short on the kind of proofs that mathematicians would insist on. Of the three, I think Sparks' book was, up to now, the best for teaching that I've seen. But after looking at Comenetz' book, I prefer it, provided that you are teaching relatively good students.
If your students just barely got through algebra, they would find this book a little rough going. Sparks' more intuitive approach might be better. But for a prospective physicist or engineer who is not too weak in math to see (and try his hand at) some proofs, I would recommend this book. One of the strong points is that Comenetz never strays far from the idea that calculus is not just slopes and areas, but can be applied to a lot of types of problems (though almost exclusively taking physics-related ones). Many people think of a derivative as a slope or an integral as an area, and fail to see that anything that can be thought of as a rate of change or the accumulation of small bits is an example of the type of things calculus handles. Comenetz tries to dispel this attitude.
As I said, this is a harder book than Sparks' book (though easier, I think, than Bell's!) But if you want to understand things on a more fundamental basis, I think this book is the best of the lot. The other three books which I reviewed all got four stars from me; this gets five. So this should clue you into the fact that I think this the best one of the bunch.