funny you mention the on your back thing. i used to be petrified of sleeping on my back but no so with this pregnancy. I think you may have proven your book to be right in that the weight of the uterus cuts blood supply off the main blood highway to your lower adomen & legs. I usually get mild BH when i roll onto my back for any length of time.. very annoying.
With the calf leg cramps i used to get them in puberty and now i get them in pregnancy. DH has woken up to me crying in pain in the middle of night, now he gets up and pushes my foot straight (my calfs are no longer stretched) and the cramp goes away.. if he didn't i'd still be there in pain trying to move my foot. My muscles are sore for a few days after, nothing a nice rub down or warm bath won't fix
i seem to remember reading somewhere that in pregnancy you need more salt and not to cut down on it(a belly belly article perhaps?)