Most of internet-based resources do not consider morphologic traits, but try Flora of China http://www.efloras.org/florataxon.aspx?flora_id=2&taxon_id=119570,
maybe you can find old books
Poyarkova, A.I. 1939. Apple-Malus Mill. Flora of the USSR. V. 9. Moscow-Leningrad: Publishing House of the USSR Academy of Sciences. 362 pp. (In Russian)
Some information about Malus taxonomy is in attached paper.
A. V. Forte et al., Phylogeny of the Malus (Apple Tree) Species, Inferred from the Morphological Traits and Molecular DNA Analysis Russian Journal of Genetics October 2002, Volume 38, Issue 10, pp 1150-1161
Dichotomous Key to Rosaceae to Malus. It has five species. Please visit the following link and you will get all five species of genus Malus. These are not Asian