If you only have your book as a PDF with a text layer, you might try a tool like Softmaker's
FlexiPDF, which are able to do an export to various formats (including HTML and ePub), and usually (in contrast to most other tools of this kind) does a great job with the paragraphs (taking only the "real paragraphs" in account, not the pdf-induced line-by-line-"paragraphs").
If the price seems a little steep, a company called Ashampoo has a version of the same tool, which usually are sold at a lower price (BTW. the free version of FlexiPDF are not able to export, so don't bother to try that).
Of course, as others have said, there's no real turnkey solution for converting pdf to epub (or probably any other format to any other format).
(And for those suspicious/wary: I am not in any way affiliated with either of the above companies.)
Regards,
Kim