If a book is freely available online, such as a public-domain book available through Google Books, there is no need to add the proxy prefix to the URL.
Google Books provides a link option for easy copying of a stable link from any page of a book. Find the Link above the book page, on the right.
The simplest method for finding stable links for online books is to use the library catalog. The library catalog provides a link that you can copy and reuse.
1. Search for the book in the library catalog.
2. Right-click on the link in the Holdings information and select Copy Link Address or Save Target As to copy the URL. This is a stable link. The URL will include the proxy prefix for off-campus access, for example:
Catalog screen showing link location: