“On the quantum theory of radiation” by Albert Einstein

In Einstein's 1917 paper “On the quantum theory of radiation” he introduced the concepts of stimulated and spontaneous emission of radiation, the effects that make possible lasers and many other fascinating devices. He accomplished this in masterful fashion by starting with the simplest of assumptions, all but one of which were traditional classical physics. Using just one basic quantum notion - Bohr's idea of quantized molecular energy levels - his amazing powers of deduction led him to hypothesize new observable physical phenomenon as well as rederiving Planck's radiation law in a very neat way. This was apparently also the first time that anyone realized that photons should carry momentum as well as energy. This English translation of the original paper is beautifully written. Here are two nice retrospectives: Einstein as armchair detective: The case of stimulated radiation by Vasant Natarajan; Rereading Einstein on Radiation by Daniel Kleppner.
A dim recollection of freshman physics is probably enough to follow much of Einstein's train of logic.
"A Theory Should be as Simple as Possible - but not Simpler"