Originally Posted by JoeD
For old series, they may not have the rights to publish the eBook or have yet to get around to converting them. But I agree it's very annoying.
An example of the rights thing.
With Dana Stabenow's Kate Shugak series the author has the rights to books 1-6, 8 & 9 herself and Minotaur/Macmillan has the rights to 7, 10-18.
Now in this case things are OK, or will be soon, because the author has hooked up with some folks to put out the books she has the rights to (she tried self pubbing them, but results/formatting wasn't great) and soon all of them will be out (DRM free too) and Minotaur has all the ones they have the rights to out and available. This wasn't the case not that long ago though and as time goes on other popular series/authors are likely to do something similar to Ms. Stabenow or to work a deal with their current publisher to get their backlists out. Even though ebook availability has come a long way from even a couple years ago it's still going to take time to get stuff out.