I'm going to make the assumption that when you say more readable, you mean better comprehension and speed.
Eric Michael Weisenmiller wrote his dissertation on A STUDY OF THE READABILITY OF ON-SCREEN TEXT (very lengthy and links directly to the PDF). It is a little dated (July, 1999) and does not focus on ebooks (at least as far as using ereader devices, although they are mentioned in passing in the conclusion), but it did try to
...determine whether sans serif and serif typefaces optimized for
on-screen viewing significantly improve reading rate and reading
The conclusion goes on to say that basically there is not much of a significant difference and there are too many variables to consider for reading speed and comprehension. For example, the time of day (how tired the person is), the subject/content (how familiar the person is with the material), reading time limits (slower readers have less comprehension if forced to read within a time limit) and a few other factors.
As for my own opinion, I believe that eInk devices will tend to be more readable in fonts that are traditionally better for printed books (there are some studies on this already), whereas tablets/phones/laptops will be more readable in fonts that are traditionally better on-screen (in the normal LCD (or CRT!) sense).