It sounds like you are seeing the tripane within the nav pane and just the topic area.
Assuming you have checked the book is linked correctly to the default topic and not somehow to the start page, I would create a new book and drag the topics to it, then delete the problem book.
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Yes, the book was linked correctly to the default topic and not to the start page. We actually fixed the problem that very day by inserting a page, as against a book, as the first topic in the ToC. Then we moved the other books to the left so what we had was a topic (the Welcome to XYZ sort of thing), and a couple of books in the ToC.
I decided to post a query here out of sheer curiosity. Had never seen such a problem and the 'why' is still niggling at me.
No, we didn't have the same problem when we restored the ToC to its earlier format. Nor did we have a problem when we created another book and moved all the topics under that. Since we were used to seeing a Welcome topic first as a page, then books, if any, we chose to fix the problem with a page first, then the books.
If I do however want to check what was wrong with the initial book, are there any specific files that I should look at?