So, I've been creating a system that can detect a writable book with specific text. I found that this system gets a little weird when it comes to the Off-hand slot.

My command block seems to be able to detect a writable book in the Off-hand just fine. Having my command block detect TEXT in the book is a different story.

When I take a writable book from creative and put it straight into my Off-hand, then write the text I have specified, the command block is unsuccessful to detect the change. Although, there are some odd exceptions.. If I go into my inventory when the writable book is filled out with it's specified text, the book does the kind of refresh animation that happens when the data of an actively held item is changed. The command block is then successful. Another exception I have found is when I write my specified text in a writable book that is in my inventory and it is detected by the command block, the book with it's text will still be detected when put into the Off-hand.

I found this whole situation very odd. In theory, the game should be able to detect text written inside a writable book that is in the player's Off-hand like any other inventory slot.