The Saxon Math Calculus book has a very unique structure: rather than being organized into "units" that focus on related topics, it mixes all of the topics from two university semesters' worth of calculus together, into 148 "lessons", each of which is meant to be finished in roughly a day.
The idea is that wrapping up everything into units allows for the learner to forget how to do the problems once the class has moved on to the next unit. This can be a problem especially in Calculus because a standard course "units" might include topics like limits, derivatives, integrals, and infinite series; each of these has a distinct flavor and with only a little bit of overlap.
For example, a common pitfall might be that after weeks of doing derivatives and integrals, a student forgets how to do harder limit problems which might not come up in derivative/integral problems.
The Saxon approach is to sprinkle in limit problems into homeworks while the class is learning about integrals and derivatives, so that later on they will be fresher in the learner's mind.
I learned from this book, and I really think the approach was helpful.
I think it particularly shines in either a high school or self-study setting, where there is sufficient time to take a day or so to do each lesson.
Unfortunately, it just isn't structured to fit the university semester/quarter system very well:
There's no well-defined boundary between Calculus 1 and 2 topics (Maclaurin Series start in Lesson 50, while the Fundamental Theorem of Calculus doesn't come until Lessons 95-100),
A single semester is so short that it wouldn't be practical to do one lesson a day and finish.
I wonder if someday there could be a more university-friendly textbook that taxes a Saxon Math approach...