I used to think our CFO was a really nice man. He seems very normal, respectable and professional – that’s why I was so shocked when I heard that he had been arrested. Apparently he’s been cooking the books (Bilanz fälschen) for years and making the company look much more profitable than it really is to keep the shareholders happy and artificially inflate the value of the company.
This idiom comes from an old way of using the word „cook“ to mean changing something to a different form, which was in common use in the 18th century. We no longer use it this way, but have been left with the expression „cook the books“