This is going to be tricky. I'm not aware of any specific books or articles in Polish, let alone English, though it's fair to assume that there must be some material available in Polish and I simply don't know about it.

The place I've come across the most material relating to the Polish volunteers is in the work of Gerben Zaagsma, who recently published a book on Jewish International Brigade volunteers (Jewish volunteers, the International Brigades and the Spanish Civil War). His work has a heavy focus on Poles as the only Jews to form a specifically 'Jewish' unit, the Naftali Botwin Company, were part of the Dabrowski/Dombrowski Battalion. I've had a quick browse of his bibliography though, and turned up almost nothing in any language which is specifically about the Polish volunteers more broadly. If something significant in English existed, I suspect Zaagsma would have found it.

If you're looking for an overview of the International Brigades themselves, Michael Jackson's Fallen Sparrows is well-written and evocative, though a little dated and with a bias towards English-speaking volunteers, so I can't promise how much specific information you'll find on your grandfather's unit. While there's a lot of primary source material available online, you'll struggle to make much headway without some extra languages (Polish, Spanish or Russian I suspect would all be useful). Material in English, predictably, relates mostly to English-speaking volunteers.