In 9.2, Blizzard introduced Click Casting. The button to activate the Click Casting Interface, gets you to your spell book, with an extra window next to it, so you can easily bind the spells/abilities (and even Macros) you want to bind to specific button or button combinations. However, some dumb ass intern at Blizz, thought it was smart to hide that button in some option panel, instead of putting it right there, in the spell book...
So this AddOn is an extremely light weight, 21 line code fix for that and it puts another button right there in the spell book, where you would expect it to be.