Hi cm, No there isn't a restriction on how much black coffee you have, as long as you have your 2.25litres of water as well, be careful I foun i was have waaaay too much (15 cups...yikes!) and then not being able to sleep!
oh actually your right the book says the 2.2litres can include black coffee n tea, i thought it was the water plus cal-free liquids, i've must have been having about 5 plus litres a day...haha! worked for me though!!