
I don't know if you have plans for a Calculus book, but I think pedagogically you've hit it on the nose here, and would like to see what the next step looks like.
Things I love about this textbook, and would like to see in a Calculus text:
1) Awareness of how much depth is valuable:
 Many concepts get presented with genuine rigor. I really enjoyed, for example, the initial settheoretic presentation of functions, as well as the illustration of how synthetic division actually works.
 I'm not bogged down in abstraction that takes me too far from the point. There's an understanding of what kinds of proofs are likely to interest me at my current ability level.
2) Intelligent construction of problem sets:
 I think you've found the right balance of how many problems to assign to a given concept (or a given slant within a concept). As I continue to work through the book, I find myself constantly learning things that I did not expect from the variety of tangles that are composed into particular problems, even when a group of problems is ostensibly geared towards training a known skill. I think very few of the problems are redundant, and I haven't found myself wishing I had more to practice with either. I'm generally confident that doing all of the problems at the end of a lesson is close to the optimal amount of practice, which is almost never true of textbooks.
3) Laid back, clear prose style
 You're funny.
 You're good at using plain language to outline the contours of complex ideas.
If there are plans to move forwards with any other open source texts, I'd love to know about it.
