For SHS your best bet is Billig's Grosses Handbuch der Fälschungen vols 35 and 36 (by Otto Stiedl). This is rather elderly but there are several more modern contributions on SHS in Jugoposta vols 2 and 35 by Roy Dehn and Don White. Specifically on the Croat Independence Stamp, there is quite a bit in Jugoposta vol 98 by Zrinjscak et al.
If you are more interested in the WW2 issues there's a certain amount spread through the Arbeitgemeinschaft Jugoslawien. I can dig out an index to that if you need it.
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