I used Jan Klabbers and Malcolm Shaw. Both are very good books. It is recommended to use Shaw as he is not only recommended by FPSC, but sometimes you need to frame your answer with a certain definition/perspective, and in such cases you need to have an authority to back your answer.

Jan Klabbers' is a very recent book. The book's 2nd edition came out last year. It is very reader-friendly. Cambridge is the publisher. I had an easy time reading this book, and it also broadens your perspective when you read from 2 different sources.