The Galaxy Tab 2 doesn't support moving apps to external SD card. Supposedly there are some apps that circumvent this, but they require root. For the time being we are stuck on having our apps, and attached media files in the internal memory. External memory is strictly for video, music and photos. Why Android doesn't include eBooks in that category is beyond me.
There is a rumored Jelly Bean update in the wings, perhaps this will solve the problem?