I do know of some books on none Japanes Kempo systems, however they are generally hard to find.
As for Shorinji Kempo books I know of a few... but I'm not sure of the availability.
There's a couple of books by Taika Seiyu Oyata that may be of interest, he is the headmaster of Ryute (Ryukyu Kempo). I belive these are mostly on the history of Ryukyu Kempo and his training philosophies.
There are also books by George Dillman & William Durbin.
