It's an expected behavior as explained in Searching | Joplin where:
notebook:bookslimits the search scope withinbooksand all its subnotebooks.
The only way I know of to ignore descendants is with the - operator. You can use a filter like -notebook:example in conjunction with notebook:books to exclude the example subnotebook. However, this can quickly become cumbersome.
A way to limit scope to a single notebook, whether it's a separate filter or the new default behavior of the notebook: filter, would be nice to have.
It could also work with other feature requests like this one: Checkbox next to search bar to limit search to current folder