One thing you can do is to download a book from the Mobileread library. Three Men in a Boat is one often cited for fancy formatting, but any of them will likely do.
Open it in Sigil and see if you like how it is laid out. Look at whether the paragraphs start with anything other than <p>. If they don't then double-click on the stylesheet in the style folder and look for p followed by {. The stuff between the brackets controls how paragraphs will be displayed.
If you like it, replace the p instructions in your stylesheet with a copy of the p instructions from the MR book and see what happens. You don't need to save in order see the effect when you switch back to book view.
If you created the stylesheet yourself, then be aware any little mistake can make all the instructions after it have no effect in the book, as if they didn't exist.
While your book is loaded, go to Tools, Validate Stylesheet and it will tell you if the stylesheet is lacking errors which can cause it not to work. Note, this is not a validation of what you mean to do, only that is correct to do it that way. No computer yet has a "Do What I Mean" key.