Online search leader Google Inc. is becoming more bookish. Expanding a program introduced last year, Google is inviting publishers to include entire books in its index, enabling people to peek at the contents before making a decision on whether to buy.

Although entire books will be scanned in, the new feature won't let people read them entirely online. But participating publishers must allow people to read at least 20 percent, said Susan Wojcicki, Google director of product development.

Book listings, which include title, author and number of pages, will appear at the top of Google's main search results page.

Books already have been submitted by more than a dozen publishers, including Penguin, Wiley, Hyperion, Pearson, Taylor & Francis, Cambridge, Chicago, Oxford, Princeton and Scholastic.

Wojcicki would not say how many books are in Google's search engine, but said the Mountain View-based company eventually hopes to scan billions. Initially, only English-language books will be indexed, with other languages added during the next few months.

As it broadens its book-search capabilities, Google may become more of a threat to online retail giant Amazon.com Inc., which offers a similar service.

When someone views a book through Google, the company will provide a link to where it can be bought online. Google won't receive a commission, but the company plans to post text-based advertising links related to the books' content.