If your device is rooted and you have a recent enough version of Android, you can try using "Link2SD". The app itself is pretty easy to use, but it requires that your SD Card be formatted in a specific way. It's fairly straightforward if you know how to manage partitions with Linux, but a Google search found several tutorials like
this one that can be accomplished with downloadable Windows tools.
If you haven't, you may want to try Mantano reader. I originally used Aldiko as my Android ereader app, but switched to Mantano because it doesn't make copies of books that it imports. Both are based on Adobe's rendering engine, so look and feel are quite siimilar.