You Died: The Dark Souls Companion features a number of easter eggs and references hidden throughout the book.
Note: Most symbols and all secret messages are part of the hardcover edition published by Tune & Fairweather. The paperback edition also features symbols (there’s some overlap with the hardcover edition) which are mentioned but no encrypted messages as far as I can tell.
Some easter eggs are mentioned in the Kickstarter campaign like the rose window on the cover being a replica of the one found in Anor Londo.There are a number of secret messages encrypted in the endpapers using a symbol substitution cipher. Also the main contributors’ initials are hidden in the Tune & Fairweather logo.
The last page features a collection of partial letters that can be decrypted to reveal a secret message. This mirrors the secret message in Soul Arts: Presented by VaatiVidya and the secret message in Abyssal Archive.