I recently picked up a great book at the Tate Modern on portraiture. Unlike the usual “how to…” books this focuses less on techniques (although it does share examples on techniques) and more on experimenting with different types of portraiture.
The author has developed the book off the back of an initiative she established called Portrait Party hosted on Flickr – https://www.flickr.com/groups/portraitparty/. Where artists get together virtually to draw each other and share their work.
You have to apply to join and membership requires you to be both a subject of portraits and a creator of portraits.
Its definitely worth checking out the site and if you can get your hands on the book its a great addition to any studio!