title_body: How can I merge multiple ebooks in a single file? Is there a way to easily combine multiple ebooks into a single file, that will collect all of them?
upvoted_answer: Is it possible to use a full fledged editor like Sigil. It works with .epub files, but if needed, before the editing, it's possible to use Calibre to convert the source ebook into .epub, then reconvert it after the job. 
  
  Anyway, for a faster and easier solution, Calibre has a very useful additional plugin called EpubMerge, that, as the name imply, can merge multiple epubs into a single one, also letting create a new table of contents and edit all relevant metadata of the new output file, while leaving unaltered the source ones. Also in this case, if another ebook format is needed, a conversion is required after the editing since the plugin only works with .epub files.