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hpr1946 - this is for Windows, and probably Linux - not sure about OSX.
Calibre uses the de-facto standard shortcuts to select multiple items. Ctrl+A to select all in current list, and Shift+Left Mouse Button click to select individual items. You can also use the Mark Books feature, (Marked Books get a gold pin in the line number cells) - Ctrl+M to Mark/Unmark books, Ctrl+Shift+M to show Marked Books, Alt+Shift+M to clear Marked books.
Selecting a single book in the book list is also done in the normal way, use the cursor keys to navigate to the line so that's it highlighted. Then do what you want to do - Convert, Send to Device, Shift+F10 to get the context menu etc.
The calibre specific keyboard shortcuts are documented here ==>>
Keyboard Shortcuts. They may vary dependent on Language setting in
Preferences->Look and Feel. Most are user configurable via
Preferences->Keyboard shortcuts.
BTW - I changed the title of the thread to better describe its content.
BR