https://www.amazon.com/Elliptic-Tales-Curves-Counting-Number...
It's not a textbook, which is both good and bad. In my case, it did a good job whetting my appetite for more!
A really well-written and not-extremely-difficult undergrad textbook on elliptic curves:
https://www.amazon.com/Rational-Points-Elliptic-Undergraduat...
(Non-affiliate links, just so you know.)