What operating system are you running?
NookStudy is kinda buggy, doing a clean reinstall of the app should do the trick. Also noted, which isn't written anywhere, you have to have an Adobe account to read most eTexts. While NookStudy is somewhat capable of creating an account for you, its best if you just head there now, register, and sync the account with the NookStudy app. This may be where most of you are getting error messages? Adobe Digital is one of the major malfunctions which is holding-up the progress of NookStudy being accessible through PDAs, Tablets, and Smartphones.