Suggestion

My choice...

Fighting the System with a Book (and a Cup of Coffee): Socially engaged bookshop-cafés in a multi-ethnic, post-Yugoslav city; or: Towards a framework of discreet civil society

While "socially committed" or "engaged" might describe a person seems like a bookshop would self describe more as radical. It's certainly what I would expect to find bookshops like Bookmarks (SWP) being described as.

this distinguishes it from mainstream booksellers and Amazon. Many independents could possibly be classed as "alternative" but does it say that this shop is "radical" or for "activists"?

Also "un auteur engagée" is seen as politically aware and/or committed but not necessarily activist or radical

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Note added at 13 hrs (2016-02-05 10:26:03 GMT)
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I think "independent" implies commitment to the local community, having a knowledgeable staff who truly know about the books they sell, hosting events instore, welcoming browsers etc . For me it would go well beyond what a chain/major bookstore will do. I just think "activist" and "radical" is overtranslation. Here are some examples:

http://www.politics-prose.com/our-history
"...We are selective about the books that we order, offering what is important to our customers and what is important to us. We sell books about the nation and the world, about people’s lives and their ideas, and we love fiction. Our staff tries to read as much as we can so that we can recommend new books. It is the enthusiasm and commitment of the staff that make the store unique...

Our author talks and our publications are integral to everything that we do. We want to facilitate the relationship between books and authors and readers. Authors enjoy coming to P&P because the staff treats them with respect and the customers are exceptionally well informed. It’s a great chance for authors, who work in solitude on their books, to meet readers. Our customers enjoy the author talks and the constant activity of the store. It’s a lively atmosphere..."

http://www.broadwaybooks.net/

http://www.newstatesman.com/culture/culture/2012/10/cabinets...
"Where Paris protects its bookshops with subsidies and by fixing trading prices, London seems intent on catering for Starbucks and other corporate chains. I for one hope the indies’ emphasis on community and expertise saves them from ruin..."
http://www.jarndyce.co.uk/bloomsbury_booksellers.php

and here's a more "radical" independent
http://fiveleavesbookshop.co.uk/