In my opinion, this is John Hick’s tour de force. In The Fifth Dimension, Hick systematically lays out a rational way of understanding how the world religions, and religious experience in general, can fit together from a pluralist perspective. I have quibbles here and there with Hick, but really the only thing I don’t like about this book is the title.
Reading Hick back to back with Marcus Borg’s Reading the Bible Again for the First Time would be a great introduction to ideas surrounding religious pluralism. If you’re looking to dig deeper into Hick, I’d recommend The Metaphor of God Incarnate as a next read.