The generations of Intel's integrated graphics
You want to look up "Intel Graphics Developer Guides", they are quite thorough.  Here's a link to the latest:

https://software.intel.com/sites/default/files/managed/22/6d/6th_gen_graphics_api_dev_guide_dec_2015.pdf

And here's the architectural Cliff Notes for the past five generations of Intel GPUs:

"In several important respects, the 6th gen Intel Core processors' graphics hardware architecture is similar to that of 5th generation Intel® CoreTM processors"

"The graphics hardware architecture of the 5th gen Intel Core processors is similar to that of the 4th gen processors."

"The graphics hardware architecture in the 4th gen Intel Core processor is similar to that in the 3rd gen processor."

"The new generation of microarchitecture, codenamed Ivy Bridge, provides another jump in functionality and performance over Sandy Bridge microarchitecture."

"With the introduction of the Intel microarchitecture codenamed Sandy Bridge, the graphics processor has moved onto the same die as the CPU, and is now referred to as “processor graphics”. In addition, processor graphics has enjoyed numerous architectural improvements that yield significant performance improvements over previous generations of Intel® integrated graphics parts."