I am trying to export a book to offline from Lightroom4 and get the following message: "Some photos in the book are offline. If you continue, these photos will be omitted from your book" What does it mean?
Look at the photos in Grid view and you'll see question marks in the corners. That shows that the original photos are no longer in the location recorded in LR's catalog - you've moved or renamed them in other software and LR doesn't know where to find them now. If you click on the question marks, you can relink the photos with the originals at their new location, and then it'll work.