Have you definitely restored your catalogue, the lrcat file? You haven't created a new catalogue, have you? Or reimported photos?
Is the book saved as a collection? Could you be opening a folder, rather than that collection?
One way to lose text that is to change the size of the book or sometimes the page layout. It doesn't always happen, but I have seen it. Try selecting a page to which you definitely added text and view it in single page mode. If you can, select the text boxes with the mouse - under the Edit menu is an alternative way to select text boxes. Review the positioning - just in case it's in the wrong place. Review the text colour, just in case it's white on white page or black on black etc. Maybe something will be obvious.
Definitely didn't create a new catalogue or reimported the photos, but I'm not sure how the book is saved. Pretty sure each photo in the book simply references to where each individual file is stored, but I have no idea where the text or the formatting for the book is saved. Does LR6 create some sort of file to save photo book formatting, etc.? I'll check to see if maybe it's a viewing/formatting error. Hopefully something will jump out. Thanks for the help!
When you take some pictures into Book the first time, at the top right of the central panel is a button "Create Saved Book". You do need to press this. It creates a special type of collection, a "book collection" which stores all your layouts and text. It's in the Collections panel, and has a different icon.
I don't think there is a formatting or viewing error. It's more likely you have overlooked this.
I have the same issue though I created a saved book. This happens with text and images... Just spent 3 hours yesterday creating a book that is now half gone.