The best complete source on this can be found in Darrigol (I will check spelling if this is incorrect.): From C-numbers to Q-numbers. Prepare yourself for a complete exposition of blackbody radiation. Your strengths better be in electrodynamics and statistical Mechanics. After you read this you will regard Planck to be mentioned in the same breath as Einstein, Newton or Boltzmann. If you have a hard time with the citation of the reference above, I will write back if necessary.