Werner's diary is an important item in Angel of Darkness. Werner Von Croy has written down many important things that will help Lara to solve the mystery around the Monstrum that has been killing people in Paris and Prague. Also all of Lara's finds will be written down.
Lara will get the diary from Werner's friend Margot Carvier, in the level Carvier's Apartment. If she gives it to Lara voluntarily or if Lara has to take it, depends on the conversation you are having with her. If Carvier refuses to give the Werner's Diary to Lara, it can be found in a cupboard in the kitchen.
Whenever there's a hint available in the Diary, a special icon appears to the lower right corner of the screen.