Dressage Movements

I was wondering if we could devise a rough guide as to the dressage movements in order of difficulty. Now, obviously some horses may find other things easier than some of the movements which will be very varied but maybe some sort of rough idea? I am very interested to learn about all the dressage movements.

Also, could this include things such as the balance of the horse, the 'correctness' - more the classical facets of the discipline as well?

Classical dressage uses the following training pyramid:

1 - Rhythm - that is, getting a horse nice and relaxed and moving in a nice forward manner (not rushed, not tight) with clear and regular gaits.
2 - Suppleness - general and lateral flexibility, flexion and bend
3 - Contact - NOT headset, but acceptance of the bit and willingness to reach for the contact
4 - Impulsion - good forward movement, coming from the back end of the horse with the quarters engaged and back legs stepping well underneath the horse
5 - Straightness - true straightness, where the whole body of the horse is in line with the line being ridden
6 - Collection - very light forehand, lowering of the croup to produce high short steps

Using this, the piaffe is one of the last things you teach (we're talking years away here), along with things such as the canter pirouette and passage. Begin by teaching your horse to go lightly off the aids (ground or ridden) remain relaxed in its movement and develop muscles correctly for balance and rhythm.

For an easy to use guide - just have a look at dressage tests, in Australia its the Equestrain Australia website that will list them. In the US I'd assume your equestrian federation would show them.
The tests are ridden in order or difficulty in training. In Australia we start with preliminary, which is basic walk, trot and canter, with some long diagonals, 20m circles and basic lengthening of the trot in the final preliminary test. Gradually the levels increase in complexity, ending at Grand Prix which contains extremely difficult, complex movements such as the piaffe, passage and transitions between then, along with single tempi changes and very steep half pass angles, with pirouettes etc. etc. etc.
For a highly experienced, professional rider on a super talented international horse, they can have the horse at GP by around 8 years of age. Most 'normal' people with an extremely talented horse will be lucky to have a GP horse in it's teens. Not all horses can reach this level, very few in fact will even make it to the FEI levels at all, beginning with PSG.

I think a book on dressage would be a better choice. And, it really isn't the "movements" that horses are tested, in a large degree. It's more the quality of the way they move. For example, in the rock bottom test, up several levels, there are movements that consist solely of a 20 meter circle at trot. But, it's the quality of the trot that is being modified and "tested". So, from the intro level "working trot" , which has the hrose with it's head more out in front, to the Medium trot, which is more on the bit/connected and then trot work while "chewing the bit and stretching downward" and then eventually collected trots and more. All are on a 20 meter circle, but each is testing further development of the horse in his gymnastic ability to maintain his balance while on a bend and working neatly forward.

There are just a gazillion books on dressage. This will be really helpful.

Depends how involved you want to get Tiny ;)
I think the OP is just after the basic step by step movements up the levels. Adding quality of paces, connection of the hind legs to the hand, etc is going to confuse them if they are not already fairly knowledgeable about dressage, which going by the question asked.. I'd say they're not :)