If you can share the laptop drive on the desktop, then at the desktop:
- use the Switch/create library->Use the previously existing library at the new location option to 'register' the Laptop Library
- switch to the Desktop library
- mark the books you want copy to Laptop Library (you may need to add the Mark Books tool to a toolbar/menu)
- show marked books
- select the marked books (Ctrl+A)
- use the Copy to library->Laptop Library tool (by default it will be in the book list contest menu)
- wait until copying is finished
- use the library tool Remove option to 'deregister' the Laptop Library from desktop calibre, that won't change the library in any way.
Use of the Mark took is a suggestion, you may have another way to filter the books - e.g. via an #onlaptop/On Laptop custom Yes/No column.
BR