If you're book will be saddle stitched, you don't need a spine, as a stapled book doesn't have one. Since it's a one sided cover, how you create the document depends on how the books are being reproduced. If the covers are run separately and being merged with the insides (either in production or post), you can just create an A4 landscape doc, with the back cover on the left and front cover on the right. These can be printed, then merged with the insides. If the covers are part of the insides, you're better off creating a 2 page A5 doc (facing or otherwise), with front cover / back cover as the 2 pages. Then, this PDF can be merged with the insides and the copier operator will designate what to print cover on / insides on. This is how my machine runs - I load coverstock in one tray and insides in another. Then through the driver, I specify how to run the book. Bottom line, you need to speak with the person producing the book. They can best tell you how they want the file.
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