Another Scalar book: The import tool assumes that this kind of import will be occuring into a fresh (new) destination Scalar book, therefore pages or relationships that might exist in the destination book are not checked for possible updating and persistence, respectively.
A CSV file with text pages: If the CSV file contains a column for main text content (sioc:content), then the import tool will not check to see if that page already exists in the destination Scalar book, because there isn't enough information to inform matches between existing and new pages.
A CSV File with media pages: If the CSV file contains a column for media file URL (art:url), then the import tool will check to see if an existing media-page exists in the destination Scalar book, based on a match of the URL field. If there is a match, the existing page's relationships will be preserved.
Book
Book properties: No attributes of the source book are transfered to the destination, because exports of Scalar books do not contain book information but rather page and relationship information.
Pages
The following attribute isn't presently transfered: Page visibility, because hidden items are ignored.
Thumbnails & Background image & Background audio: These fields can link to media uploaded directly to a source book, which can cause problems should the source book be deleted (more below in Media section).
Media
Imported media: Any media imported to a source book via an outside archive (ie, the media has an external URL) will be properly transfered.
Uploaded media: Media uploaded directly to a source book will not be physically transfered to a destination book. Rather, the destination media pages will point to the URL of the media housed in the source book. This means that in the event that the source book is deleted, the media link will go dead. However, this problem can be corrected using the "Replace existing" feature at Import > Local media files.