You can currently download the eBook The Best Things in Life are Free from Lonely Planet for free. In it you will find tips on what you can do in 60 metropolises around the world as cheaply as possible or even for free!
The information is definitely not bad and is always loosened up with tips or rankings, e.g. Europe’s best Tours by Public Transport.
Unfortunately, the eBook is only available as a PDF and not in ePub or PRC format.