Copyright of web pages and web sites is a legal nightmare, not least because most companies are quite happy for screen shots and images to be taken, if they are used to support and promote their site. Of course, when they are not, the issues are extremely complex.
For those sites that allow me to define what and how I want information, who does the copyright over the finished look belong to? Have the site owners not just provided me with a development tool to produce my own site? If I take a screenshot of a site in a specific state, because it is dynamic and changing, then is that instantaneous image something the providers should have rights over?
On the whole, if you are being fair and reasonable about the screenshots you use - that is, not misrepresenting the sites or the organisations - then you are probably on acceptable legal grounds for "fair use". The real difficulty come when, as I suspect, you are wanting to critique sites, and make negative comments. You are probably still on "fair Use" grounds, as long as your criticisms are reasonable and valid, however as @JedOliver pointed out, if the site owners decide to make a big fuss, then, irrespective of the final results, your book publication could be delayed, which might be serious.
If you can, contact the copyright holders, where you can find their details. It may be that such a critique will prompt them to change things, and you can then comment on the changes too. But at least they will know what you intend to say.