To cite your PDF books in Mendeley, you can follow these steps:

Open Mendeley Desktop and click on "Add Document" button.

Select "Add Files" and locate the PDF book you want to cite on your computer.

Mendeley will automatically extract the metadata information from the PDF file (title, author, publisher, etc.). If the metadata is missing or incorrect, you can manually edit it by selecting the document and clicking on the "Edit" button.

Once the document is added to Mendeley, you can cite it in your papers by using the Mendeley citation plugin or by copying and pasting the citation information.