This is a purchase site, but you can download the app online for free books. Www.eprint-studio.com works great on my q10 (I'd assume any other BlackBerrys that can download apps from online) along with other android style phones.
I took a bit different route, I downloaded Snap (android apps directly downloaded to your BB)to my Q10 And then downloaded Kobo. It works like a charm, and with Snap you can download a lot more goodies.