Note that you have four separate sources of documentation.

ARM has general documentation about their cores, the instruction set, timing, cache coherency, ...

The compiler vendor have documentation about the compiler, debugger, ... and often a lot of nice sample code.

The chip manufacturer, who have used an ARM core and then bought or developed peripherial functionality that have been glued to the core. There is a huge difference in quality of documentation, and quality of peripherials between different manufacturers. But new Cortex chips has more contents directly from ARM, which should reduce the difference between the best and the worst.

The evaluation board manufacturers. Some just send out a board. Some have large amounts of documentation and sample code.

ARM is ARM, so it's basically your only source of information for the core or instructions.

NXP normally have very good documentation for their ARM processors. They often have very nice peripherial functionality too, but not always at the best price.

Keil normally have very nice-to-work with evaluation boards, and access to evaluation version of their compiler. The eval boards are good to work with, even if you decide to use some other development tools.