Hopcroft & Ullman[1] is generally widely recommended. I only just got my copy a couple of days ago, so haven't had a chance to dig in yet.

The Sipser book[2] is also generally recommended as being very good.

[1]: https://www.amazon.com/gp/product/0201441241/

[2]: https://www.amazon.com/Introduction-Theory-Computation-Micha...