The recent books on Einstein by Isaacson and Neffe (see reviews) cover the development of special relativity in great depth, with special attention to the development of an operational definition of time duration that vanquishes the prior notion of simultaneity. Indeed, simultaneity is now accepted as observer-dependent, thanks to Einstein’s Special Theory.
Both books do refer to the role played by Poincaré- although "role" is not necessarily the operative term here. No doubt Poincaré was thinking deeply about time, but did not make Einstein’s leap to codify the relative nature of time duration.
An excellent book on the Einstein/Poincare connection is Einstein’s Clocks, Poincaré’s Maps: Empires of Time, by Peter Galison. Here Galison tries to relate Einstein’s experience as patent clerk who most likely saw patent proposals for clock synchronization between different Swiss cities to his ongoing thought -experiments in the foundations of physics. ( In a generally very favorable review by R. Wald for Physics Today, this idea is called into question.)
But the Poincaré connection is fascinating because it is not clear to what extent, if any, Poincaré’s writings on time were even seen by Einstein. (And the title of the book is itself a play on words, because a Poincaré Map is a fundamental analysis/visualization tool of chaos and fractals.)
The new Einstein bios have really opened up new avenues of thought on how Einstein came to be Einstein. The Poincare connection is fascinating, and a potentially important piece of the answer.
Most amazing of all is the pictures these books paint of the intellectual ferment taking place in Europe at the end of the 19th and beginning of the 20th century. It is hard to imagine a similarly exhilarating time of new, earth-shattering theories that can possibly duplicate these years for excitement and creative brilliance.