gPDFText is a text editor that opens PDF documents for ebook readers, converts the text contents into plain ASCII text, restores the original paragraphs, and removes unwanted line breaks to allow easier zooming on the reader. The text is loaded into a text editor with spell-checking support and can then be saved as a reformatted PDF in A5, B5, or original A4 size.
mdepub is a tool that allows you to compose a book in Markdown format and use Pandoc and Calibre to compile an Epub package including all of the book's source material. In effect, you can keep the source and product in the same file in your library; if you ever want to revise the product, you merely need to extract the source, make edits, and recompile.
Book Manager is intended to provide a simple interface to manage a collection of e-books. Users can also view their e-books, as well as many other files in tabs. With the collection disabled, Book Manager can also be used as a tab-supporting shell for the popular KDE PDF reader Okular.