Title. A few people have posted about this, but I figured out precisely what they did.
There are three kinds of view on google books: full view, snippet view and preview view. Full view is only for public domain books. For in copyright works, you have preview and snippet. Preview lets you see some full pages while snippet view lets you see a chunk and some sentences.
There was some change a month ago that knocked out a lot of books from search, and I am not entirely sure what that change was. But a few days ago they removed ALL search functions for any books with previews, which are disproportionately modern books. This has made Google Books effectively useless. Snippet view remains and is still searchable.
Bizarrely, if you check the individual books that used to have search function enabled - the previews are still there! You can read full pages! However, the search/OCR is completely turned off. You can search a word while staring at a page that has that word and it will tell you 0 results. They also did not turn off OCR/search for snippet view AND completely locked books.
I think this is extremely terrible as it makes finding books to buy extremely difficult. It also clearly isn't for copyright, as the previews are still there. I have no clue why they did this.