WorldCat is a community catalog containing records of book collections from around 72,000 libraries in 170 countries. This also includes Danish libraries. Here it is possible to find most books and export references to Mendeley
Here you can choose to use the web importer or if you just need a single reference:
Click on Cite / Export and in the box that appears, click on Export to Endnote / Reference Manager.
Then the file is saved and when it is downloaded, you drag it into Mendeley depending on which browser you use - see under Import / Browsers