This has become a very popular question lately. I've gotten a number of emails from people who have read through plenty of introductory Wiccan and Pagan material and have no idea where to go next. Nobody seems to be able to find 'advanced' books on Wicca.
Think about it like shopping for a cookbook. There are thousands of beginner cookbooks on the market, but you rarely see books titled "Advanced Cooking".
Once you get past the beginner stage, most people begin to branch out.
That's when you start buying more precise cookbooks, like pastry baking, BBQing, candy making or beer brewing.
I feel that is a good direction when studying Wicca, Paganism or witchcraft as well. Instead of trying to step upwards, think about walking sideways instead. Think about non-general topics that interest you and delve into those.
Here are some suggestions for further study, as well as some links to help you get started. The material I have here is still quite beginner-level but it gives you a direction to start off:
The Sabbats - Take a deeper look into the holidays of the year. Really get in touch with the reasons for celebrating and the age-old traditions that are still followed today.
Practical Skills - Why not put the books aside and learn what you can directly from the Gods themselves? Improve your meditation skills, try to spend more time doing rituals in sacred space, or master some form of divination (Tarot, runes, scrying).
Non-Wiccan Paths - Even if you are comfortable with Wicca, you can still learn quite a bit if you take the time to explore some other Pagan beliefs, like Santeria, Druidry, Celtic or Greek paths.
Hands-On Crafts - Try growing your own herbs, blending your own oils, making a deck of Tarot cards or sewing a special ritual robe. These may seem trivial but knowledge can be found even in light-hearted tasks.
Of course, these are only some suggestions to help you advance your learning. You may find that you are drawn in different directions entirely.