Hello. I am looking for learning materials for the field of number theory. I took a class this semester in number theory and the topic fascinated me, but I don't feel like I learned that much. I am a mathematics minor so I have a very strong background in basic math, abstract/linear algebra, and calculus, but in number theory I don't get much beyond Diophantine equations. So, do you have any book recommendations? My only requirements are that the books are inexpensive (so no current textbooks) and that the answers to at least some of the questions are in the book. I don't want to have to Google for answers to every question in any chapter. Thanks for the help!