I recomend you a book by Thomas de Waal "Black Garden: Armenia and Azerbaijan Through Peace and War" and by Tatul Hakobyan "Karabakh Diary: Green and Black". Other book is "The Nagorno-Karabakh Conflict: A Legal Analysis" by Heiko Krüger, but I don't know this book.
And we have two books about Nagorny Karabakh, but only in polish:
1) Abkhazia, South Ossetia, Nagorno-Karabakh: the genesis and functioning political systems: oryginally: author Rafał Czachor, "Abchazja, Osetia Południowa, Górski Karabach: geneza i funkcjonowanie systemów politycznych", Wrocław 2014, ss. 443, ISBN: 978-83-935729-8-4
2) Armenia and Nagorno-Karabakh in the processes of social and political transformation: oryginally: "Armenia i Górski Karabach w procesach transformacji społecznej i politycznej", Wrocław 2014, ss. 182, ISBN: 978-83-935729-5-3