a lot of these things are pretty simple tips. like, sloppy oral is just intentionally make yourself gag a couple of times because it produces extra saliva. Deepthroating is often as easy as just relaxing your throat, because gagging happens when you're not relaxed and your throat is trying to get rid of something it feels blocking it.
That said, I don't buy books on sex so I can't vouch for how good they are. They could be great.