That's strange. According to Google support, the feature was discontinued with the new Google Books interface, with plans to eventually bring it back.
Link? What's the link say? I'm used to radio box that lets me choose between searching just within My Library or all of Google Books.
At the top, when logged in, I have the search box and the Advanced Search link, but that doesn't allow me to search just my library.
And then I have the new bookshelves feature as the main content.
And finally, on the left, I have a sidebar that has my name, city, a "map" link, then icons for websites in my google profile, including using the favicon if there is one.
Then the next block down is My Library and has these links
* Reading now
* To read
* Have read
And that's it. No search within My Library.
If I'm not logged in and looking at My Library, I get a list of subjects to browse as well.