How did a culinary book give him away?

They'd just been discussing the revelation that the killer was eating body parts, including the thymus, with Will suspicious of the idea that Hannibal (a man who he doesn't believe makes many mistakes) hadn't thought of it. Then he finds that a book has been left out, containing a highlighted thymus-themed recipe. Hannibal had asked if Will had shared his information with the bureau, and left the room. It was too suspicious to be a coincidence, really.

I myself am wondering what the odds are of a cultured man like Lecter doing something as vulgar as vandalising a classic book (with something very basic, that could potentially implicate him in a crime, no less). Not a great decision on the part of the filmmakers.

Brett Ratner isn't known for making much sense in his films. In Hollywood he is on the list of directors to higher when no one else will do the job. He's a hack and lame director

I forgot how it was in the book, but in Manhunter, Graham notices Lector has a 'Wound Man' book. A book that graphically shows a human bodies with different wounds. Kind of a reference book as I understand. An odd book for a psychiatrist (or anyone) to own. Graham presumably didn't even have to open the book to make the connection (a book an FBI agent is probably familiar with).

Actually I had that reversed. The book has the "Wound Man" thing, and in Manhunter he says he saw Lector had a book of 'war wounds'.

You just have to be resigned-
You're crashing by design

I thought it was a human anatomy book that he had marked a section with "sweetbreads" implying that whatever anatomy he had that written by would be a the part of the body he wanted to eat.

Dragonzord! Mastodon! Pterodactyl! Triceratops! Saber Toothed Tiger! Tyrannosaurus!