I would echo what some others have already said.
Good conformation is good conformation and good miniature horse conformation is no different than good full size horse conformation. A nice, comprehensive book on general horse conformation will serve you well.
The curve ball, if there is one, has to do with type.
What I think has helped me to improve the quality of my herd has been to get out there and show, and honestly stand back and look at what is doing well in the ring. Then, very thankfully, I've been blessed to have a good friendship with a mentor who has given me valuable advice over the years. She is someone who is consistently, year after year, bringing home National championships on horses she's bred. Very knowledgable and with a gift for explaining things in a way that's easy to understand and remember.
There's not a shortage of people willing to give advice about how to select a miniature horse and what to look for, be that here, privately, at shows, in books, etc. However, not everyone is equally qualified. When it comes to taking mini horse advice to heart, I'd look at what the person has accomplished in the show ring and with their breeding program before deciding how much of my heart to hang on their advice. Anyone can say what "you should do", even if they haven't actually known what they themselves should do to
If you can get your mind around good basic conformation and then for the type, look close at the breeders and farms who do consistently well at the National level, you will be on your way to identifying quality in miniature horses.