To the best of my knowledge theoretically most obfuscation techniques are also applicable to libraries, but there are considerable downsides: Slowdown and increased support-difficulties. Please note if a program does something, there is always a way to find out how it archived it when you control the execution environment.
I'd advise you to keep any business logic you want to protect out of libraries by keeping the libraries generic. If your business model concerns the distribution of logic you want to keep a secret - its a pretty bad situation.
If you really want to offer some confidential functionality and performance is not an issue, think about offering it on the server side.