Jagose's book is definitely a good place to start as they said, and I wouldn't recommend starting with Butler and Sedgwick. Foucault is more readable but also not an easy read.
Spargo's book Foucault and Queer Theory is a great place to start. It is a very readable book and you can find it here: Foucault and Queer Theory (monoskop.org)
And if you have some money to spare, this is the best intro to queer theory I have ever read: Queer Theory Now: From Foundations to Futures | Amazon.com.br
It is an overview of queer theory and how it developed, and it has a lot of reading suggestions and explanations of concepts that have been developed by queer theorists. Unfortunately, I can't find it online anywhere.