Wandering Tinker's Diary is a tome in Act I of Diablo III. It is needed to complete the achievement A Quick Study and it can be found in the Sheltered Cottage in the Fields of Misery.
It reads as follows:
We should have known. The farmers left over a fortnight ago, and yet we stayed. Last night, we heard the beasts creeping closer. My husband left to investigate, and I haven't seen him since... All I have left are the things we made together. Hopefully they are of more use to someone else.