A long while back I bought the omnibus of the Han Solo Adventures, three books from 1979-1980. I remember my dad had originals of them but I hadn't ever read them. I finally got around to it this year. The books are not written all that well and obviously there's not a lot of continuity because only one movie existed at the time. Still, they were enjoyable books about Han and Chewie and the Millennium Falcon before they ever went to Tatooine.
What was really great to me is these were technically prequels but they weren't written like prequels. By that I mean they weren't written with the purpose of trying to give Han Solo some grandiose origin. They weren't written to hide a lot of "Easter Eggs" for fans. They were just Han and Chewie flying around doing shit--and that was perfectly fine with me.
There's going to be a Han Solo prequel movie and I'm sure it's going to do all the lame prequel stuff like tell us about Han's childhood and give him some wonderful destiny and settle important questions like where Han's vest came from. I'm sure Boba Fett, Jabba the Hutt, Greedo, and maybe even Lando (if someone at Lucasfilm decides to remember he exists) will show up. They'll probably go into how Han met Chewie and got hold of the Falcon. There will probably be references to Darth Vader and Princess Leia and all that.
These three books don't deal with any of that (because most of it didn't exist yet) and I didn't miss it at all. Because I really don't give a shit about where Han Solo came from any more than I care about where Wolverine (and his jacket) came from or where Darth Vader came from.
It's the kind of thing I wish they'd done with the new Star Trek movies instead of that lame, implausible origin story in the first one. Or if Better Call Saul instead of trying to give him some pathetic origin had just focused on some outlandish cases he might have done before getting involved with Walter White.
I think I've said before that the problem with prequels is they're always trying to add significance to things that don't need it (Wolverine's jacket) and give every character some "tragic" whiny backstory to explain why they are who they are. But I don't know, would you rather know where Han Solo's vest came from or watch him pull some con job and escape some Imperials in the Falcon?
So like I said in the title, the best prequel is one that isn't written like it's supposed to be a prequel. In other words, a story that's written to show the character in action instead of trying to explain a familiar character. Take note, Hollywood!