When looking for a good reference to back the “many-core problem” assertion in my Master’s thesis, this the best I could find as a prime source. Multicore: Fallout of a Hardware Revolution holds an excellent description of the reasons behind the shift from increasing clock speeds to multiplying the numbers of cores in modern CPUs.
“Hidden concurrency burns power
Speculation, dynamic dependence checking, etc.
Push parallelism discovery to software (compilers and
application programmers) to save power”
…and a hidden treasure of information on the history of all modern processor architecture optimization techniques.