first of all, CONGRATS! that sounds really cool. imo communication with the other artist is key. taking some time to find out what you both bring to the table, and what both of your visions are, will help a lot. get familiar with his other work if u arent, and encourage him to do the same!
get contracts, get payment arranged in writing. if anyone says "we don't need a contract, u can trust me," that's a pretty big red flag, but im guessing things are on the up and up there. for basic Industry(tm) stuff, one of the managers of
Heroic Records has a lot of good resources. basically, now that there's money being exchanged, it's a good idea to treat it like a job, albeit a job that you enjoy a lot.