[Rachel]

I access my Calibre library in my Dropbox folder on my android tablet. I open Dropbox, go to my library folder which holds all the author folders in alpabetical order. I navigate to the author I want (the hardest step as there is no index running down the side as there is on my ipad version of the Dropbox app), open the folder and then choose the book folder I need. Once that is open I select the file (epub in my case, but you might be using a different format depending on your reading app). Dropbox then asks me which app I would like to open the file in. I select Aldiko and the book downloads very nicely, ready for me to read. None of this changes the files in my Calibre library, so doesn't pose problems.

Having said all of that - I agree with eschwartz that creating a calibre2opds catalog is better because it has more organisation than the Dropbox folder ie you can search by author, title, tag etc. I would always do that for preference, but sometimes I want to read a book I haven't yet added to my catalog, and then I go through the process I describe above.