The U-Boot Documentation
This is the top level of the U-Boot's documentation tree. U-Boot documentation, like the U-Boot itself, is very much a work in progress; that is especially true as we work to integrate our many scattered documents into a coherent whole. Please note that improvements to the documentation are welcome; join the U-Boot list at http://lists.denx.de if you want to help out.
Driver-Model documentation
The following holds information on the U-Boot device driver framework: driver-model, including the design details of itself and several driver subsystems.
U-Boot API documentation
These books get into the details of how specific U-Boot subsystems work from the point of view of a U-Boot developer. Much of the information here is taken directly from the U-Boot source, with supplemental material added as needed (or at least as we managed to add it - probably not all that is needed).
Architecture-specific doc
These books provide programming details about architecture-specific implementation.
Board-specific doc
These books provide details about board-specific information. They are organized in a vendor subdirectory.