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
https://www.nxp.com/docs/en/user-guide/UM10204.pdf
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.