The only book I know of on the subject is: Dierama; The Hairbells of Africa by OM Hilliard and BL Burtt. It's not readily available. If you find someone willing to swap, grab it!
I purchassed "The Harebells of South Africa" in RHS Wisley bookshop two years ago. You can contact them through the RHS website and enquire if it is still available.