Check the official device tree website.
device tree source files (.dts) are compiled with dtc (device tree compiler) to flattened device tree BLOBs (.dtb files). Such .dtb files can be read and manipulated by the flattened device tree library libfdt - which is part of the dtc project too.
To install all necessary stuff on Debian/Ubuntu/Raspbian :
sudo apt-get install device-tree-compiler libfdt-dev
FreeBSD has some documentation which might be useful if this is a commercial project. Last but not least the most experience on ATAGS parsing on the Pi is probably concentrated in the bare metal forum on raspberrypi.org.