The best way to start using dropbox for your books is to copy them all to the correct folder in dropbox (hope this is the correct folder: apps/books; after you have granted access for syncing reading position it should be there). Then you can load books directly from dropbox. I am not certain if the following works correct, but if you import a book in one device, it will also automatically be copied to dropbox. And it should then also appear with the recent books on other devices.
Experiment a bit if this last feature works. It should.
I have bought the paid version and do not regret it one minute. I have used Aldiko for almost two years, but there are too many issues that simply are not tackeled by Aldiko. They seem too much concerned to generate extra income through book sales and advertisements.