I just checked and yes, search inside of books with previews is still possible.
(a) when you search books.google.com and find a book with a preview, it opens their new book viewer - the search is at the bottom of the page. You can also click "View All" to see all references of your search in that book.
(b) if you go to the book homepage (clicking X in the top right of the book viewer if that opened), there's still a "Search Inside Book" next to the "Preview" button under the title.
(a) when you search books.google.com and find a book with a preview, it opens their new book viewer - the search is at the bottom of the page. You can also click "View All" to see all references of your search in that book.
(b) if you go to the book homepage (clicking X in the top right of the book viewer if that opened), there's still a "Search Inside Book" next to the "Preview" button under the title.