Google Book Search: New Ways to Search Within a Book
…one more addition to the experience of searching a book: search results in your scrollbar. Now when you search in a book, little hints will appear in the margin to indicate where you results are located. When you hover over one of these annotations, you’ll get a quick preview of the search results and the option of jumping directly to the associated page.
Previously, it was difficult to get a feel for where results were located in a book. You could count the page numbers and make a guess, but that’s hardly efficient. Now there is a strong visual display of result locations, and often clusters will form around particular chapters or passages. This will help you navigate more easily between pages which contain your search term.
Source: Google Book Search Blog