Volume always seemed confusing to me until I discovered VSA (volume spread analysis). Now I find volume to be very helpful.
I initially found out about VSA during a free trial at Advanced Trading Workshop (ATW). There are 2 free videos on the website describing a few of the best VSA patterns. However, you have to register for a free 30 day trial to watch the videos. There is also an excellent MACD divergence video that can be viewed without registering, if you are interested:
http://advancedtradingworkshop.com/Freebies/tabid/98/Default.aspx.
I learned a lot during the free ATW trial, even though I did not sign up for the full course. Once I understood the basics of VSA, I purchased the original book on VSA called "Master the Markets". It can be purchased as an e-book for $9.95:
http://tradeguider.com/m90_checkout/Cust_Product_Books.aspx
The book is well written, but very detailed. It really helped to watch the ATW videos first to get the big picture and to understand a few of the most important VSA patterns.
I have read a lot about volume and have listened to several speakers in an attempt to understand it. VSA is the most helpful approach to understanding and using volume that I have found.
Of course, volume alone is no more important than trend, momentum, etc. But when a volume signal lines up with your other indications for a trade, it further improves the probability of a successful trade.
I am not a client of and am not endorsing ATW or TradeGuider, although I learned quite a lot for free from both organizations.