mentioning websites

I don't know that it would be a necessity to get permission. Most sites are out there because they want to be found. But it would be a courtesy to drop them a line, compliment their site and mention your intentions.

Good luck with the book.

You don't need permission, but make sure the sites are well-established or by known permanent organizations, and try to give the root URL. I've had links disappear on me in the month between writing and publication.