You Died: The Dark Souls Companion features a number of easter eggs and references hidden throughout the book. (Note: This is only true for the hardcover edition published by Tune & Fairweather, not for the paperback edition.)
Some are mentioned in the Kickstarter campaign like the rose window on the cover being a replica of the one found in Anor Londo.
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.