Google Books provides full, free PDF scans (and web-viewable versions) of public domain books and magazines, free previews of some books and magazines under copyright, and the option to purchase digital editions of available items to save and read on Google Play, their e-reading platform. You can also find a local library based on your zip code that has the item available. Reader reviews of the items are available and you can contribute your own. Search for authors, titles, as well as keywords and Google will return results with matches in the title/text of the item. There are permalinks to available pages. Highly useful for finding items that contain your keywords in its text.

It has the Google Ngram Viewer as part of its service. The Ngram Viewer is a service connected to Google Books that graphs the frequency of word usage across their book collection