Could you tell me the main area of the books in different libraries, for instance, library 10 is for music and ...? I would like to search books in language …

I am not sure if I fully understand your question.

You are right, Library 10 is the library that has the largest collections of music, but they also have some books, especially on IT and travelling.

In every other library in the Metropolitan area there are books on languages and arts, and also some encyclopedias. The largest collections of arts books are probably at Rikhardinkatu Library
http://www.lib.hel.fi/en-GB/rikhardinkatu/yhteystiedot/.

Pasila library is the Main library of Helsinki City Library, so there are the best collections of reference books like encyclopedias. They also have a multiligual library with books in about 70 languages, and a lot of books for learning different languiages.
http://www.lib.hel.fi/en-GB/pasila/yhteystiedot/

Whichever library you visit, the whole collections of all HelMet libraries are at your disposal. Via HelMet web library http://www.helmet.fi/search~S9/ you can reserve materials from any HelMet library to the libarary of your choice. There you can also check which libraries have the titles you need in their collections. To find out how to use HelMet web library, please read the Help pages http://www.helmet.fi/search~S9/k

If you would like to have a look at the classification system in HelMet libraries, i.e. in which shelves you can find encyclopedias and books on arts and languages, please use Web-HCLCS
http://hklj.kirjastot.fi/en-GB for Helsinki City Library and
Web PLC
http://ykl.kirjastot.fi/en-GB for Espoo, Kauniainen and Vantaa Libraries.

For example, in Helsinki libraries, the arts books are in classes 700 to 789: http://hklj.kirjastot.fi/en-GB/luokat?MinClassNumber=700&MaxClassNumber…
Books on languages are in classes 850 to 899:
http://hklj.kirjastot.fi/en-GB/luokat/?MinClassNumber=800&MaxClassNumbe…
Encyclopedias are in classes 030 to 038:
http://hklj.kirjastot.fi/en-GB/sanat/?SearchWord=encyclopedias