Kauyon contains update for Tau Codex from 2012, but it costs about double the price of new codex. It also contains rules for Tidewall Shieldline/Droneport/Gunrig and 2 Tidewall "combinations" which count as a single fortification FOC: Tidewall Gunfort(3x Gunrig) and Tidewall Defence Network(4x Shieldline + (1x Gunrig OR 1x Droneport) which are not in Tau codex 2015. I've been told the rules for these fortifications are included on a sheet if you buy them... but I'm not sure the combinations are.
Similary, Mont'ka contains rules for Dawn Blade Contingent and it's formations, which are identical to newest Farsight Enclaves suplement. With one exception, in Farsight Enclaves suplement (newer than Mont'ka) they changed access to signature systems, if a formation/detachment is of Farsight Enclaves, it can now choose from both Tau Empire and Farsight Enclave sig. systems.