From the announcement:
Open Library now lets you search inside the text contents of over 4M books!… When you search across 40M documents, it can be a challenge to find the one you’re looking for. One feature which Open Library has been missing is a way to limit Internet Archive’s full-text search to only include results from books on Open Library. So for the last two years, Open Library has patiently waited to take full advantage of full-text search for its users.