This information comes from Damian Grove, regarding Sonic 2 Hacking Notes:
I figured that this is a fairly big importance for the Sonic community and figured it probably would be good to address it in the Sonic HQ news section.
I have released my ‘unreleased’ Sonic 2 hacking notes to the public. I have held on these for a long time now (some over 2 years), but have decided to let the public eye see them. There are 21 text files all zipped up, filled with piles of information (47.2KB total — uncompressed). These were planned for quite some time to be put on one of my sites –Sonic 2 Hacking Guide, but never got around to it. So finally, I decided to release them as is for those people who wanted them ASAP. You can download the notes from www.sonichacking.org.