While looking for an article on the sun's self-absorption of spectral lines, I came across a very early review article by Feynman on path integrals. The article is in Reviews of Modern Physics: http://rmp.aps.org/abstract/RMP/v20/i2/p367_1 The article is excellent! It starts from basic quantum mechanics and explains path integrals in a very matter-of-fact, pragmatic manner. Review of Modern Physics carries fairly comprehensive reviews of a variety of physics topics.