Canons were assembled repeatedly from the fifth century onward and destroyed almost as often. The Tang and Song collections are known chiefly from catalogues. The most severe loss came under the Mongols: in the religious-political disputes of the thirteenth century, imperial edicts ordered Taoist books outside the Tao Te Ching burned, and the destruction was carried out. What survives today is essentially one object (the Zhengtong Daozang, printed in 1445 under the Ming, with a supplement, the Xu Daozang, added in 1607), and it is a Ming reconstruction of a much larger literature, not a continuous inheritance.
Even that survived narrowly. By the early twentieth century very few complete sets were left; the 1926 photolithographic reprint of the Baiyun Guan copy in Beijing is what put the canon into general scholarly circulation, and modern Taoist studies effectively begins there. The three-volume Taoist Canon: A Historical Companion to the Daozang, edited by Kristofer Schipper and Franciscus Verellen in 2004, describes and dates every text in it and is the reference work for the whole field.
One consequence deserves stating plainly. Because the surviving canon is a Ming selection from wreckage, arguments from silence about early Taoism are weak. The absence of a text from the Daozang is very poor evidence that it never existed.