Wireless is a dynamic frontier changing at a rapid pace. With that in mind I will rely on the authors of the links below to provide up-to-date information. This is by no means a complete list but it will get you started.
WAP Development Kit - UP.SDK from Phone.com
WAPman - a Palm Web browser that uses the WAP (wireless application protocol).
Building WAP Services - XML And ASP Will Set You Free by Luca Passani in Web Techniques
WAP Usability Guidelines are available from Serco
List of Wireless Style Guides - Usable Products Company
Wireless Web Phone Study - Usable Products Company (registration required)
Mailing List: Usable Mobile mailing list on http://www.smartgroups.com (registration required)
AvantGo - one of the leading
companies in this technology, using HTML rather than WML. There are some good links on
design issues for small devices