The developer of a new layout should read the xkb protocol specification (The X Keyboard Extension: Protocol Specification) at least to clarify for himself some xkb-specific terms used in this document and elsewhere in xkb configuration. Also it shows wise to understand how the X server and a client digest their keyboard inputs (with and without xkb).
A useful source is also Ivan Pascal's text about xkb configuration often referenced throughout this document.
Note that this document covers only enhancements which are to be made to XFree86 version 4.3 and X11R6.7.0 and above.