Well, I first want to say that you are so welcome here and we look forward to hearing about this new adventure you and your lady will start out on. (by the way, it's fiancee for a girl, fiance for a guy).
I can't think of a good beginners book. IT's kind of like there are so many, many books out there, but is there one that is golden for beginners? H m m m. Someone else will have a good idea.
Your approach of clarity of communication and consistency is absolutely the correct way to approach horses. Thsi not only makes them well behaved, but much happier. They are much happier when things are crystal clear, boundaries, I mean.
However, they do not train exactly as dogs do, because they rarely will work for praise , as a dog will. And, in fact, being overly cuddly with them will only invite them to see if you are someone they can push aon a bit and get the treat , without doing anything other than mugging on you.
So, horses work more from pressure and release of pressure. Think of being more of a "shepard", than pack leader. When I said shepard, I meant that you start out driveing a horse. You drive him to back up, you drive him to step over, you drive him to go through the gate . Eventually, he feels good following your "orders" and will become more of a "follower" to you, in which case you start leading the horse, who mostly willingly follows. Sorry this sounds a bit woo-woo, now that I read it. Just thinking off the top of my head.
Now, I'll pass this off to the next person!