I have read The Crane’s View Trilogy and White Apples. I liked them very much. I’m not sure if he’s at his peak, but I would start with that trilogy, and work forwards, then back. He’s written novels since at least 1980.

Also, James Morrow is another excellent novelist in a more metaphysical and philosophical vein.

What is his best book, and what you should start from, are not necessarily the same things, for any author. In his case, though, most of the books stand alone, so starting pretty much anywhere is fine.

Personally, I like his older stuff a lot, including Land of laughs. The Crane’s View Trilogy (though I did not even know it was considered a trilogy!) is good too, as mentioned, but personally I think White Apples is one of his weakest.