This functionality makes a lot more sense as an extension; it saves us the hassle of visiting/bookmarking a new site. It also saves you the hassle of building good search functionality, which is quite tricky for books if you are using titles and not ISBNs.

We put together an open-source version for this a while back, but it's been gathering dust ever since. https://github.com/OpsopiDev/extpricext

Anyone with javascript experience should be able to fork this and add niche sites they like with little work.