Many analog electronics book has a chapter on analog multiplier. Razavi CMOS design book has a small section on this under differentail amplifier section. Gray/Meyer book should have it too. And many more...search in IEEE website helps or if you can find, get the classic paper by Barry Gilbert, the inventor of Gilbert multiplier, he wrote this paper in 1968 or 1969! search on Analog Device website where Gilbert works also help. for starter, Google it to get the basics.