Ok that title needs to be put in context...
But, in many ports you can find the same old books, the basic stuff, pellews, boatswain 6-7 , rum rations etc.. and this is what I am referring to by 'too common'
I'd like to suggest that these basic books be less common, I mean you only use them once, and increase the availability of the 'basic' upgrades.
Many ports will have no upgrades at all, but they will have 7 different 'basic' books. Players use more of the upgrades than they ever will books. I'd also suggest reviewing the drop rates of these 'basic books' from AI ships, especially those above 6th rate. I'd expect most players will have collected the majority of the common books by the time they are fighting 5th rates, so perhaps they could be replaced with basic upgrades in the drop tables, or their likelyhood to drop reduced.