SpringerLink provides metrics for books via Bookmetrix, including the number of citations, mentions, readers, downloads and reviews. When viewing a book in SpringerLink, click the 'About this Book' link to the right, and then see the 'Book Metrics' information to the right. You can click the links such as: 'Downloads' or 'Mentions' for a breakdown via chapter.
Goodreads is a free social cataloguing website with a user-populated database of books, annotations and reviews, which you can search. Plum Analytics has recently expanded its support for books to include Goodreads metrics including ratings, reviews and the number of readers.
Amazon.com is an online bookstore (which also sells other products). Users can rate books out of 5 stars and write reviews. These ratings and reviews can be used as metrics, for example Plum Analytics includes the number of Amazon book reviews as an impact metric. Whilst purchasing products requires an account, all ratings and reviews are freely available.
Google Books is a free search engine that allows you to search the full-text of books and magazines that Google has scanned. Users can rate and review books on Google Books and these ratings and reviews could be used as impact metrics. In addition you can use Google Books to search for citations to other publications within the full-text of digitised books. Research suggests that Google Book citations correlate significantly and positively with Web of Science citations.
A free meta search engine for the online catalogues of libraries all over the world. You can select a geographic location to find the closest libraries that hold an item. You can use the number of libraries that hold an item as an impact metric. Research suggests that WorldCat holdings correlate significantly and positively with Web of Science citations.