It's not a book, just an article from Eye magazine no. 40, 2001.

The article is entitled "Digital Type Decade", written by Emily King.

I found it very interesting when writing a short thesis on recent years typography last month.

Good luck!

You may also take a look at this page

http://www.microsoft.com/typography/web/designer/default.htm

Mmm... it's not a book and it's from Microsoft... I know... just try!

The Eye magazine "Digital Type Decade" article is available online.

http://www.eyemagazine.com/feature.php?id=4&fid=3

Go to RNIB's website (www.RNIB.co.uk) and have a scooby around. You can buy a pack off them that details designing for the visually impaired for various media including the web. It not only outlines suitable font styles and minimum (and maximum - some people with visual impairments can't read text if it is to big believe it or not) sizes but also points out colour combinations required to assist people with various types of colour blindness and visual impairities. If you want visual clarity you can't find a better test group than the visually impaired - if they can see it people with 20/20 vision should have a much easier time of it to.

I've heard good things about this one:
Typographic Design: Form and Communication, 3rd Edition
Available at http://www.wileyeurope.com/