When you're reading a book on public transport and you get to a naughty bit.....

#Bookspic.twitter.com/gnnXaMd9yf
You can add location information to your Tweets, such as your city or precise location, from the web and via third-party applications. You always have the option to delete your Tweet location history. Learn more
it's the second best place to read it. 
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.