Adding to troops strength is just the same as what the Tower and Baths do but in a different way. It's too easy to work out if the extra pop or extra 5-10% is more effective thus rendering the weaker 1 obsolete.
What would be better is to have an extra level of research options when the library is built along with it's current increase in research points. The research options would need to be something that affect the game in different ways.
eg: Smaller increases in troop speed, Strength, Favour production that are around 5-6 research points each. This would allow you to make a smaller all round improvement compared to the single larger improvement of the other buildings.
Move the spy special researches to the top level so that you would have to specialise to get the silver bonuses.
Possibly diplomacy and ceramics could go there as well so you would have to decide whether a city is safe enough to be a resource producer or if it needs stronger military protection.
