At the top of the Books tab there is a search box. It's a great way to edit attributes in bulk.
There are two parts to it:
Search box. Typing in the search box will limit shown books to those with attributes that match what you've typed. You can search for titles or authors right off the bat. And if you've added a custom attribute called "Genre", you can type "Romance" to bring up all the books where the Genre attribute is set to Romance.
Search filters. You can click the "Add a search filter" button to add advanced filters to your search. This lets you filter by specific attributes. It add a dropdown where you can choose which attribute you'd like to filter by, and then you can select which values for that attribute to include. For example, you could filter to only include books where "Genre" is set to "Science Fiction" or "Fantasy". You can add as many filters as you need. You can also filter to only include books that don't yet have a value for a certain attribute by selecting "Not set".
At any time you can click "Clear all filters" to remove all filters and return to your full book list.