USRP Hardware Driver and USRP Manual  Version:
UHD and USRP Manual
UHD Development Manual

API Documentation

The majority of the actual API documentation is in the auto-generated part of the manual. Use the tree browser at the left to click your way through the class list, the namespaces or files. Also, the search bar at the top can be used to search for function calls, classes or any other publically available symbol in the UHD namespace.

Some additional pages on developing UHD are also available here: