In this website you can find catalogues on Chinese, German and Danish philately.
In addition, you'll find a comprehensive review of the market situation by hammer prices for which these entires were finally sold at international auctions.
In China Philately Vol.I you'll find the images of several hundreds of covers and postcards of the Foreign Post Offices in China at the turn of the century. Many of the Russian mail cancels are described in detail according to Tchilinghirian.
On 365 pages of the Vol. II are shown about 1,000 covers and cards of the Chinese Empire issues, Historic Documents, Large Dragon, Small Dragon, Dowager Empress, Red Revenues, Imperial Chinese Post, Chinese Imperial Post, Customs Mail Matter Handstamps and Paid Handstamps etc.
In Vol. III you'll find the images of 750 covers and postcards of Hong Kong, Macao, Taiwan and Tibet.
New books are "DDR Landpost" and "Denmark Royal Post" (2020).
In the link to the books you will find more detailed information and national and international book reviews.