In order to develop anything but the most trivial application using Motif, you will need additional documentation, particularly if you intend to rely on Lesstif for development. O'Reilly publishes an excellent series on using and programming for the X Window System called The Definitive Guides to the X Window System that covers everything you ever wanted to know but were afraid to ask. If you can afford only one reference, let it be the Motif Programming Manual (Volume 6A of the X series—the blue one). The cover price on these books is $34.95 US and they are worth every penny.
Motif Programming Manual (Volume 6A, The Definitive Guides to the X Window System), Dan Heller & Paula M. Ferguson, O'Reilly & Associates, Inc. is a tutorial on Motif and takes you from step zero through a thorough exploration of each powerful widget offered by Motif. It assumes you are fairly new to the world of X, which makes it an ideal tool for new systems programmers. In addition, the book contains enough information on the X toolkit (Xt) to make it vital for any Motif developer.
Motif Reference Manual (Volume 6B, The Definitive Guides to the X Window System), Paula M. Ferguson, O'Reilly & Associates, Inc. fills in the blanks left by volume 6A, and offers a comprehensive treatment of the Motif 1.2 functions and widgets. This is not a book to pick up and just read, rather it is an extremely well-organized reference. A thorough description, usage and resource list is provided for each widget and function. As you get serious about Motif development, you will find this book can really grease the skids.
A couple of excellent web sites can get you in touch with most of the available widget sets, libraries and utilities (both Open Source and commercial). Kenton Lee, a recognized expert on X development, provides a wealth of resources at http://www.rahul.net/kenton/xsites.html. This page seems to be updated fairly often. MW3: Motif on the World Wide Web at http://www.cen.com/mw3/ is updated less frequently, but has a nice set of links to other sites hosting all sorts of Motif-related resources.