Absolute Necessity.

There's really not much to go into detail on here, it helps compile all of your "guide books" from mods like Tinkers Construct into one Akashic Tome, which can transform into any book you've inserted into it via shapeless crafting.

I've found it extremely useful and time saving, as well as saving my poor inventory from the.. heaps of books that can appear in some larger modpacks. 5/5, likely to have prevented at least 5 cases of manslaughter.