You could use a a content search utility, like the one that comes with Windows File Explorer, to search your books for words and terms of interest. Drop the search results into the Drop Search Results plugin, which will Mark the books. Then add the word/term to a tag-like column (My Tag Cloud) for Marked books
If you use File Explorer Search you can save the searches (blah-blah.search-ms) as 'books', then you can easily run them again to find the words/terms in newly acquired books.
You'll probably have to convert books into a format that the content indexer can handle, e.g. DOCX or TXT.
I do content searches of 100,000+ books in < 5secs. It can take longer to add the 'tag' to the Marked books than it takes to find them
Note I'm referring to File Explorer Search not Start Menu/Taskbar/Cortana search. You may have to enable the indexer and configure it via Control Panel.
BR