On this page of the Disco Elysiumgame guide, you will find a detailed walkthrough of the Read your ledger and name the case task.
In Disco Elysium, the task Read your ledger and name the case requires you to read the document-filled ledger obtained during the Find your paperwork task and then name the case appropriately.
You can find the ledger in your character's inventory. To open the book, simply interact with it. Once you have finished reading the ledger, you can borrow a pen from Kim to give the case a suitable name and complete the task.