That warms my heart as an open-source enthusiast, but it's slightly strange, for a couple of reasons.
First, the GPL is a software license, and is less suitable as a copyleft license for books than the GFDL or a CC license.
In some cases it sounds as though the first time a word or concept is used, he's assuming the reader has already heard it defined.
I would, however, recommend this book to someone who needs to refresh her memory of calculus, and doesn't want to spend hours wading through epsilons and deltas to get to the highlights.
The approach is visual and intuitive, and there are lots and lots of graphs and numerical calculations.
I felt, however, that it took a long time to get going, and the idiosyncratic selection of topics might make it difficult to use at many schools.The book uses many numerical examples, which gives it a modern feeling .After all, calculus was invented by Newton and Leibniz because they needed to do calculations in closed form, but nowadays it's more natural to solve many problems on a computer, using a spreadsheet or a programming language.The writing of textbooks and making them freely available on the web is an idea whose time has arrived.Most college mathematics textbooks attempt to be all things to all people and, as a result, are much too big and expensive.I had to estimate page number 136 by weighing part of the book on a postal scale.Related to this problem is the fact that the book has no index or table of contents.Combined with the increasing consolidation of the publishing industry, this has blown the lid off of textbook prices over the last decade.But remember what the World-Wide Web was basically about before the Dot-Com Detour?The author may consider it a trivial task to set up a spreadsheet or write a ten-line program in Python or Mathematica, but it's not so trivial for many students, and they will need extensive guidance from elsewhere to be able to carry out such computations for themselves.This makes the text incomplete in practical terms: any instructor wanting to use it would have to come up with extensive support materials to go with it.