We actually just bought both Life of Fred Fractions and the Key to Fractions series for my 9yo ds. Looking over them, I think that they're both great and I would recommend both if you could.
If you have to pick just one for someone who is having trouble with fractions, I would probably just get the Key To... books.
Midwife (CPM, LDM) and homeschooling mama to:
yo ds 11
yo dd 9
yo ds and 7