Here a few (I have not read them, so make sure they meet your criteria/philosphy first)
Volhard Dog Training and Nutrition: Library: "Leave It" Command https://positively.com/dog-behavior/...cues/leave-it/ How to Train Your Dog to Leave It - For Dummies
And a video:
https://www.youtube.com/watch?v=asVQYYSWPJc
I always dropped one treat just out of his reach and said leave it. If Mario tried to get it, then I said no and moved him back and stated leave it again, once he left it for short periods (count of 5 or so) then I picked up that treat and gave him two treats from my other hand almost at the same time as picking up the one treat. Repeat until he got the idea down pat.
I increased time, decreased his distance to the treat and increased my distance away from him slowly. Lots of praise.