Formatting of books and ebooks

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Yet again I am struggling and if I knew the answer to this question I would save hours of time. When formatting any book or eBook I end up doing a lot of things manually because when I want a space like this space all the time, I end up with some spaces like between paragraphs. I know how to change the space above and below a paragraph, but it does not solve the problem. WHAT is it I am doing wrong? All suggestions gratefully recieved. It happens in both Libre Office and Google Docs.