For those of you who have watched the Da Vinci Code do you remember the main character John Langdon who interprets symbols? I found this book he's written on ambigrams (words that can be read upside down or from left to right) called Wordplay, they're truly amazing... You can turn them upside down and still read the word!
You can have a look at his website on:
Ambigrams, Logos, Word Art