I worked through this book several years ago and I remember enjoying its style of pointing out an interesting property of a particular function, and then showing, step by step, that a whole class of functions have that property; that is, the theorems are built up from examples, instead of the other way round. I also think each step was quite manageable - there were no big gaps where I was left scratching my head not knowing what to do. It is not meant as a reference book, as you're more likely to find sketches or hints to parts of proofs, rather than complete proofs. I don't know if it's ever been used as a text book, but if it were, students couldn't just sit back and absorb knowledge - they would have to figure things out (with the benefit of lots of pointers to tell them what needs figuring out, and hints about how to do it), which is just what the book is intended to encourage.