The following list of books are for Motif programming:

Volume 6A: Motif Programming Manual, By Dan Heller, Paula Ferguson & David Brennan, 2nd Edition February 1994 1-56592-016-3, Published by O'Reilly & Associates, Inc.

Volume 6B: Motif Reference Manual, By Paula Ferguson & David Brennan, 1st Edition June 1993 1-56592-038-4, Published by O'Reilly & Associates, Inc.

And lets not forget the Motif FAQ which can be found at:

Ohio State University, USA
Oxford University's Libraries Automation Service, UK
Utrecht University, the Netherlands

and many other locations near you.