Hello again, just full of advice tonight aren't I... don't worry I leave the country in two weeks so I'll be quiet for a while then...

not for long though...
As for legs... everybody has a different way, personally, I am of the "hang loose" school of thought... your legs should hang down the sides of your horse, not stuck out, just gently draped around your horse.
Western riders should appear to communicate telepathically with their horse, so flapping legs is a no no in my book... it's ugly for a start, and it does little other than desensitizing your horse to subtle leg cues... which is the point of western equitation. If you flap for nothing, how does your horse know when your are flapping for real? if you have to flap to keep your horse moving, your horse needs more training!
If you were to ride in a pleasure or horsemanship class with flapping legs you'd be penalised. You should sit still, and upright in the center of your horse.
Hands, depends on the rein, split with snaffle, split with curb, rommal, or mecate with bosal... but generally yes, thumbs are on top, reins dont come through third/fourth fingers as with english but pass through a closed but not tense fist.
Split reins with a snaffle are generally held bridged (both reins pass through each hand), tails of the reins must be crossed and hanging on the opposite side of the neck/shoulder.
Mecate are closed reins,used with a bosal (bitless bridle/hackamore) and held similarly to english in that the excess just drops down the shoulder.
Split reins with a curb are held in one hand, should be left if you are right handed and right if you are left handed (leaves your good hand free for roping you see!

) with the excess hanging down the same side as your rein hand, index finger of rein hand goes through to seperate the reins.
Rommal reins, also a closed rein, used with a curb, are held in a closed fist, no seperating index finger, with the rommal being held 16 inches from the rein hand... I believe, don't quote me I'll have to double check!
Rein length depends on the horse, the rider and the discipline. A finished horse and rider can operate on just the weight of the rein, so long drape of rein, like AQHA pleasure horses. Reining riders, have a reasonable length to show their horses are "willingly guided" but have a little more contact for the manoeuvers. Barrel racers/games riders/cattle work all tend to have shorter reins, partly for contact, mostly for safety I believe. Horsemanship has a shorter rein then pleasure, but with a light contact, and curbs are ridden different to snaffles and bosals, so theres no short answer to that I'm afraid!
And as for great books, get on Amazon, theres loads. All disciplines, all trainers, all styles... you'll find the ones that work for you, just have an open mind and dabble in all til you find the right one/s.