Initially the I2C Bus specification had been written by Philips Semiconductors. This company became NXP Semiconductors which now it the stakeholder of the I2C bus specification.
Unfortunatelly the recent I²C-bus Specification, NXP document Version 6.0, 4th of April 2014 is not available for public on the NXP site anymore. It had been located at
If you are lucky to have an NXP account you still can download it right here
Anyway you might find the NXP application note AN10216-01 “I2C MANUAL” quite helpful. Furtunately NXP provides this document AN10216-01 without the need to have an account.