My proof-writing skills aren't quite as good as I'd like them to be, and I'm wondering if anyone can suggest particularly good books to help me develop better mathematical reasoning. I learned this the hard way by opening Spivak and struggling through the first few chapters. I understand the calculus concepts pretty well, but I'm having trouble following the algebra and understanding how he is able to come up with these proofs. I understand proof writing isn't supposed to be fast or easy, but I don't think I'm ready for Spivak yet. I've started going through Velleman's How to Prove It, and it's a decent book, but I feel that I'm going to need more practice over the summer. Does anyone know of good books at my level?