>>2 I couldn't fin any webpages with completed transaltions of volumes 11 and 12 and other users told me that there was no way of finding them. So I ended up just reading the summaries and read volume 13.
Thank you anyways ( n . n )
I'll help you though with your request.
For reading the light novel and its second part, Durarara!! SH:
https://www.baka-tsuki.org/project/index.php?title=Durarara!!
For volumes 11 and 12:
http://mizaya.tumblr.com/post/55260769132/durarara-translation-index
Even if mizaya has volume 13 in progress, I recommend you to continue reading it in Baka-Tsuki.
Today I've just finished the first part and is AMAZING. I love it. One of the best character driven stories I have ever read.
If you have watched the anime start from Volume 4, which continues the story after the clash between the Yellow Scarves and Dollars (and Saika). It is the Hollywood arc.
Hope you enjoy it !!