I have an M90 and use it a lot but see it as more of a device for reading books than studying from them. I don't think epaper devices are fast enough for the latter use.
For study you need to flip pages and search very fast, be able to take notes, or call up a notebook/calculator/spreadsheet/dictionaries etc. instantly as well as do websearches. You also need to be able to flip to other books and skim through pages. All of these need a serious tablet, probably a full Windows tablet with stylus ability like the expensive Samsung Slate 7.
With the M90 I start a book and then stay with it until it's finished, occasionally pausing to make a note with the bookmarking feature. At that level it is excelent.