title_body: Does a “one-fits-all” file format exist or should I offer multiple ebook formats?
upvoted_answer: Virtually all e-book readers can display PDF files. Unfortunately, PDF documents are "pre-rendered" - the text is positioned on the page in absolute coordinates and text cannot reflow on smaller / larger devices (without hacks or trickery that rarely work well).
  
  TXT files are a good option if formatting isn't a concern. Most e-book readers can display TXT files (I just double-checked that they work out-of-the-box on my Kindle Touch). The downside, of course, is that you cannot use any formatting.
  
  This leaves you with two common formats:
  
  
  EPUB (supported by Nooks, iBooks, Kobo, and Sony devices)
  MOBI (supported by Kindle devices)
  
  
  This page displays a table of devices and their supported formats.
  
  I would actually suggest a completely different approach - produce content using a markup language (like Markdown or HTML) and then use a tool to convert the files to the e-book formats you wish to support. Tools like Calibre support nearly every format under the sun, so you can choose as many formats as you want.