You'll probably be fine, but without knowing what phone you are using and whether you have an SD card in it, hard to be 100%.
The average book is probably a 3 megabyte file. So if you'd be able to hold 300 or so books per gig of free space on your phone. Assuming you have a new-ish OS on the phone that uses onboard storage pretty intelligently.
also, you don't have to download your whole library. I have an android device with a library of books available to it, but only a few actually downloaded to it, and it works out well.
As a point of reference, my Nook for Android app has 90 books in it which take up about 250mb. Magazines, however, are much larger. My 22 mags take up about 700mb. Your Stratosphere has 4gb Rom built-in.
When I installed the app, every book showed up with its cover, but they all had a Download label on them, so I just touched the ones I wanted to download.
Thank you roustabout and petesnook! I will download my nook app today on my phone!