Which library in Helsinki's city center has the most material in English?
The National library of Finland has the largest collection of material in English. Undergraduate library has also quite a good English collection. Both of these are open to all the customers, you don't have to student or scholar to borrow books from them. Their material is mostly scientific.
The Helsinki city center has also city libraries, which have English material also. The largest English collection in city center is in Rikardinkatu library. Pasila's library has larger collection, but it is situated a couple of kilometers from the city center.
Is you are searching for music and cds, you should go to Library 10. They have the largest collection of cds in Helsinki area.