A lot has been written about the Barbary Corsairs
I hope you can read French because most has been published in that language. In Italian and Spanish too.
To assess their importance, I'd advise you to check
all the articles by Alberto Tenenti, he assessed precisely the impact of the 'corso' (ie piracy) on Venice's maritime trade. Fortunately, you can read his conclusions summarized (with maps) in Fernand Braudel's masterpiece The Mediterranean and the World of Philip II.
B. and L. Benassar, Les Chrétiens d'Allah (a very good book and rather ideology-free based on the records of the Inquisition, a lot of maps and tables and graph too so even if you don't speak French you may get somethings from it)
I know Robert C Davis also wrote something on the subject but I haven't read it.
The French historiography on the subject is very expansive. I particularly recommend M. Fontenay (some articles on English I think, check on scholar.google.com) who explores the corso as at a Mediterranean level (inclusing the orders of Malta and San Stefano and the very informed views of D. Panzac (maybe some articles in English) who sees piracy as the only option left to seafaring people in North Africa confronted with the competition of European merchants and fishermen.
Finally in English I recommend Molly Greene's articles and books (one I sumarized here: http://premodeconhist.wordpress.com/2009/02/12/greene-m-2000-beyond-the-northern-invasion/)