Istoričeska geografija na severozanadnite bălgarski zemi ot sredata na 4 do kraja na 1 v. pr. n. e. (Po danni ot monetnite nachodki). Historical geography of the North-Western Bulgarian lands from the middle of the 4 century B. C. till the first century B. C. - Belitov, Ilija (1991)