Rannye bolgary i finno-ugry v Vostochnoĭ Evropy - Khalikov, Alʹfred Khasanovich; Khazakov, E. P. (Evgeniĭ Petrovich); Khuzin, F. Sh (I︠I︡ALI im. G. Ibragimova KN︠T︡S AN SSSR, 1990)