It's 18+ years old now, but I still find myself dragging it out for rehab ideas.
"Childrean & Sports Training" by Jozef Drabik, PHD, 1991.
Doc Drabik is a Polish P.E. professor who spent plenty of time in the trenches. From page 1 you can tell he knows what he's talking about. A lot of times now I find myself scratching my head & wonder if some of these academicians ever spent one day in a gym or classroom.
This is a cheap book that will give you tons of ideas.