This is just a "shoutout" for what I feel is an excellent source for F&SF middle length stories in ebook format.
GigaNotoSaurus publishes one story a month, anywhere from long short story to novella length. It's edited by Ann Leckie (of
PodCastle fame).
With just 18 stories so far, one has already received a major award nomination,
"Sauerkraut Station" by Ferrett Steinmetz nominated for Nebula's "Best Novelette" category. This speaks well of the editor.
They post for online reading and have ePubs for download. The ePubs' convert to MOBI with no problems if you use calibre.