libx. This is a free and open source browser plugin developed at Virginia Tech by Prof. Godmar Back and Prof Andrew Bailey. It is available for chrome, firefox and a couple of other browsers.
Editions specific to a lot of universities across the world are available. Even if you do not belong to any university, you could load one (it does not require any credentials for basic usage), e.g. you could load the Virginia Tech edition (simply because of my assumption that it is likely to be comprehensive. There are students still working in the research group to continuously extend it using VT's library catalogue as baseline).
When you navigate to amazon.com, e.g. their listing for the
C programming language, 2nd edition book here and scroll a few lines down to the product details section, you can find the ISBN. Now, the
libx plugin makes the ISBN a hot-link automagically. See picture below.
Now, simply clicking on the ISBN hotlink shall take you to VT library's OPAC (online public access catalogue). See screenshot below
Clicking on the link takes you to the detailed record in the catalogue. On the right-hand side of the page you get the
cite link, as well as ability to export to a variety of formats (including
bibtex among countless others).
This may sound like a convoluted way in comparison to the other dedicated tools listed here, which I partly acknowledge. However, the
libx tool works not only in amazon, but also among lots of other sites and databases, such as
compendex -- basically anything with a ISBN, ISSN or any other sort of citable content.
It is a swiss-army knife for citations. Considering its academic lineage and its free and open source nature, it is my go to tool.
Full disclaimer: I am a former Virginia Tech student and some of my friends were involved in developing this software as part of their MS and PhD dissertations.