Library data can be exported from the portal database by the using Data export plugin available under Data and Export.
For creating a new export, Click on the New button on right top of the home page( ref screen shot above), it will open up an export creation page that looks as follows. Select Book options( to export library books) from export and the format type( either CSV or XML format) to schedule the export.
Please note that, the export option you have selected under exports will be available again only after 30 days from the date of export.
You can view the jobs and their status from the scheduled job page. Click on the link shown below and keep on refreshing the page till the status turns to be download.
After all jobs are successfully completed, you will be able to download the file from the download link given on the data exports home page.
For more clarification, please watch this.