I don't know Humphries.
Steelman's is all right, but nothing to be very excited about. As with most sight-reading books, it does not tell you very much and consists mostly of several pieces/passages in progressive order of difficulty. It will no tdo any harm, but why not get one of the several anthologies of piano music and sight read through those instead?
As I mentioned a number of times, the really good book on sight-reading (the best I've seen so far) is Howard Richman's "Super sightreading secrets" (Soundwise)
Best wishes,
Bernhard.