Shape is immensely important in Go, and you can read about the basics anywhere - don't make empty triangles, table and mouth shapes are good, etc. Where can I find more advanced treatments of shape?
I'd only offer a few more suggestions over those given so far.
The delightful book by Kageyama, Lessons In The Fundamentals Of Go. Besides being an excellent all round resource and pleasingly written, it has a chapter, ch8, on some fundamentals of shape. It may be too basic for your skill level, but it is a must read for any Go player in my opinion. Heres a link to the details for this book on Senseis Library. http://senseis.xmp.net/?LessonsInTheFundamentalsOfGo
There is also the Rob van Zeijst, Richard Bozulich book Making Good Shape, which is volume 3 of their "Mastering the Basics" series. It is a shape problem book, and I find many of the problems to be challenging.
Finally, I would look at pro games, both new and old. They are full of examples of good shape, and exploits of bad shape. Both books above have pro games in them to review.
Hope that helps.
Another good way to learn shape is to play shape game against a stronger player. Stronger player plays white you play black. To start with black places stones along the 1st lines, one space apart:
White starts the play, with the aim of making ONE living group in the center; black's aim is to not allow this to happen. Playing this game is fun and teaches beginners about good and bad shapes naturally.
Charles Matthews wrote an excellent book on this topic, coincidentally called Shape Up!
It is available for free on the net, if you have a Gobase.org account simply grab it here, if not, Google should bring up something. To list a few of the ideas explained:
Part 1 of the book, "Principles of development", covers basic terminology (table shape etc.), wedge weakness, empty triangles, eye shape, tactical aspects of connections, one point jumps and compound shapes.
The second part "Principles of engagement" explains hane at the head of 2/3 stones, nose tesuji, bulges, the center of 3 stones, answers to attachments, open skirts and submarines.
Part 3 "Practical fighting" deals with all kinds of cuts, light play, keima, peeps, various pincers, caps and sector lines.
In Part 4, "Vital points and shape in the opening", 2 and 3 point extensions and their invasions, 3rd and 4th line and enclosures are discussed.
The final part talks about "Theory", pretty advanced concepts like 3 instead of 1-2-3, miai, kikashi, probes and large distance jumps, haengma and sabaki.
Interestingly, this book can be understood by beginners (high DDK) and is still useful up to lower dan levels. All shapes explained are extremely common in actual play so studying this book is most rewarding. There are also several sets of tsumego to reflect on the chapters.
Sadly the amount of higher level books on advanced shape study is limited in the Western world. If you're capable of speaking Chinese, Japanese or Korean you'll find a vast amount of very high level books in Asian Go stores.
Apart from joseki study and Asian books, you're pretty much out of luck regards of lecture material. Of course there are still some high level (Western) books and lectures but they are rarely focused on the idea of shape itself, so you'll have to scrape together what you find.
As you grow beyond the point where the typical Western shape books are of much use, the study of joseki is the main way to learn. Joseki are carefully crafted by professionals over time, and the shapes they incorporate are often very advanced.
However, once study material has become difficult to get, you'll probably have connections to several sensei that will be able to help you out. Until then, let the basics become natural, and review your games to understand and learn from your shape mistakes.