I have both "Encyclopedia of Chart Patterns" and "Trading Classic Chart Patterns" by Bulkowski. I have found them both useful. He is the only guy I have seen who provides a statistical significance in the construction of the chart.
If I was going to buy just one of these books I would go for the latter. It covers a significant proportion of the other book, plus the other book was looking a little dated but I notice there is a 2nd Edition that may remedy that. I don't have the second edition.
I have trawled most of the books in the book shop behind the London School of Economics looking for a better book and I have not come across one that has made an attempt to prove the significance of the charts.
I think this book is relatively inexpensive for the information it contains.