bbTablet

This is not industrial strength code, but it should be sufficient for a quick first-pass prototype, or research application. The source code is available and is completely free so you can add to it as you find yourself needing more access to the underlying Wintab API.
bbTablet is quick and dirty, and easy to use. You can go from no tablet support to support for basic pressure readings in next to no time. It also has the unique feature of being able to give you a 3D rotation for the orientation of the brush for tablets where tilt is available (Intuos and Intuos II series of Wacom tablets only, I think). This requires more effort than you might think.
bbTablet - bare bones digitizing tablet interface C API for Win32 The API translates most of the tablets integer values into single-precision floating point numbers in the range [0,1) (type float). So you dont need to worry whether youve got a pen with 512 levels of pressure or 1024. Zero is no pressure, 1.0 is full pressure. Similar for positional values: bbTablet also reports those to you as [0,1) floats.