Not a textbook, but the NIH's "Gene Reviews" have really great summaries of all of the genetic diseases. The geneticists I worked with use them all the time to look up anything about a given condition. It's like an UpToDate for geneticists (UpToDate itself is awful for genetics stuff)
http://www.ncbi.nlm.nih.gov/sites/GeneTests/review?db=GeneTests
Search for the condition there and then click the "Reviews" icon that comes up after the listed name.
Here's an example of the entry for Hunter's Syndrome:
http://www.ncbi.nlm.nih.gov/books/NBK1274/