Here's a list of resources and references for further information on Tuva and Tuvan stamps

James Negus, Philately's Ugliest Ducklings:   Rehabilitating the 1934-36 Issues of Tannu Tuva.  The Philatelic Journal July/September, 1960 v50, n835-837, p 54-63.  Fantastic article documenting what it took to get Gibbons to correctly list Tuvan stamps, and lots of primary information.  Also see "Bibliography of Tannu Tuva" in Philatelic Literature Review v17 N4 p 194 for a list of 66 published Tuvan articles and books (many of which are elusive).

Andrew Cronin has written many fascinating articles and papers on Tuva, but especially see Cronin, and Stephen, W.S.E:  "Tannou Touva:  A Survey" serialized in The Journal of the Rossica Society of Russian Philately #54-59,61,66 which pulls together most of the information known about Tuva and still has detail that is not represented in any book (eg possible plate flaws on later Tuvan issues)

Blekhman Authoritative handbook of Tuvan philately, translated and updated by the TTCS and available through the Tuva Trader. 

Friends of Tuva (FoT) wonderful group of Tuva nuts (philatelic and not).  See their web site.

Tannu Tuva Collector's Society an affiliate of the American Philatelic Society serving those interested in the stamps of Tannu Tuva.  Responsible for publishing Tuvan catalogs and the translation of the Blekhman handbook. 

SilverDalen Stamps - Anders Backman's home.  He publishes the Tuva 2000 catalog and documented the semi-official Tuvan stamps made in 1994-5 under contract to the Tuvan government.  He has a very interesting plan to update the catalog on the internet so your catalog never becomes obsolete.