Notebooks for Mac 2 provides a combined full text search and document filter in the footer of the document list (the second column).

Filter the Current List
As you start entering text in that field, Notebooks filters the currently displayed list and displays only the documents containing the given word or phrase in their title. This is handy when you look for a specific document in a long list: enter a few characters that are contained in its title to quickly turn up that document.
The filter is available in regular books, but also in most of the smart books (even recent items and recently modified.) It operates on the list of currently displayed documents, so it does not look at the contents of nested books.
To clear the filter and return to the regular list, just clear the text field.
Full Text Search
If you type return after entering the text, Notebooks starts a full text search. The scope of the search is by default limited to the current book (and the hierarchy below), but a checkbox right next to the search field allows you to change the scope to all books (that checkbox shows up as soon as Notebooks displays search results.)
The search results appear as a list of documents, similar to the contents of a regular book. Notebooks does not display any preview for the search results, but displays the documents’ paths instead.
To clear the list of search results just clear the search field.
How to Search the Contents of Notebooks 2