Two of the books he found were cross-genre mysteries. The PI in these books is a zombie. Unfortunately, he found books 2 and 3 and wants to read book 1 first.
A brief description of the series at TV Tropes:
A horror-comedy detective series by Kevin J. Anderson, chronicling the cases of Dan Chambeaux, private investigator in the Unnatural Quarter. Shot dead while seeking his girlfriend's murderer, Dan returns to "life" as a zombie: one of countless "unnaturals" spawned by the supernatural upheaval of the Big Uneasy.The series is also described at The Thrilling Detective Web Site.
Some of the other books he found:
One book that intrigues me is The Case of the Toxic Spell Dump by Harry Turtledove. I have not read anything by this author and this is also the first time my son has tried a book by Turtledove.
Foundling is the first book of Monster Blood Tattoo, a children's/young adult's fantasy trilogy written by Australian author, D.M. Cornish. It tells the story of Rossamünd, a boy unfortunately christened with a girl's name, who has lived his entire life in a foundlingery (kind of an orphanage) before he is chosen to become a lamplighter in a far away city. The book's action takes place entirely on the Half-Continent, a Dickensian world run by arcane science and alchemy, and plagued with deadly (and not-so-deadly) monsters. It also won Best Young Adult Novel at the 2006 Aurealis Awards.
I personally don't go much for young adult fiction, but the best young adult fiction is enjoyed by all ages, so I may give this series a try one day. My son has read the first in the series, Foundling, and did like it. You can see from the photo above that the books get longer as the series continues, but each book has a good portion of back matter to explain the world and the terminology. There are also some very nice illustrations by the author sprinkled throughout the books,