The Kuzari: In Defense of the Despised Faith, by Yehudah HaLevi, is considered a classic Jewish book. It is well-known, so I won’t go on very long about the topic of the book.
This is an essential read for anyone who wants to get a clear picture of HaLevi, the great medieval Jewish poet and intellectual superstar. His one extended work on strictly intellectual and religious topics, and in prose, this work is a must for anyone who wants to get a glimpse into HaLevi’s mind and world.
His concerns will be remote to the average reader. He spends a great deal of time arguing against the legitimacy of the Karaites and Jewish philosophy. Hot button topics at the time, but extremely remote for modern readers. And this is not spellbinding reading, even for people interested in such arcane matters.
So, by all means read The Kurzari; get it under your belt. But realize ahead of time that you are entering another world. And you will have to do some hard work while there.