It is, in fact, his last novel, and quite a diversion from his usual fare; not only is it not the science fiction he was known for, it was written from the viewpoint of a single, female narrator, both aspects which suggest that he could have broken into mainstream literature, had he not died shortly after completing this novel.
Also because of PKD's premature death, The Transmigration of Timothy Archer has unofficially taken the place of the final volume in the 'Valis Trilogy', Valis and The Divine Invasion (more of which at some point in the future).It is, however, only thematically linked, and very much a stand-alone story.
The initial premise of the novelis that some newly-discovered scroll frgaments cast doubt on the authenticity of the gospels. Already controversial bishop Timothy Archer (based on Dick's friend, outspoken bishop James Pike) subsequently goes on a journey of questioning his faith, risking his job and livelihood, and having a romantic subplot along the way.
While it does make for an interesting look at the nature of faith, the overall tone of the story is pretty depressing. It definitely isn't a sales pitch for any kind of religion, so one to avoid if that's likely to bother you. Various story events are also loosely based on the life of James Pike too, but that won't detract from Dick's story unles you are already familiar with the life and work of Bishop Pike.
On the other hand, if you like the storytelling and/or philosophising of Philip K Dick, it's an interesting deviation from his usual fare and shows another side to his talent.
PKD on amazon