Hermann Kern's book
It was originally the catalog, in Italian, of an exhibition on the labyrinth, prepared by Kern and presented in Milan in 1981. The book contains the inventory of all the labyrinths and documents known at the time, with photographical reproduction of almost all items. An augmented edition in German was published in 1982: "Labyrinthe -Erscheinungsformen und Deutungen - 5000 Jahre Gegenwart eines Urbilds (Labyrinths - Models and Interpretations - 5000 years of presence of a primal image), another one in 1983. It was reprinted in 1995.
The English updated edition was published in the fall of 2000: "Through the Labyrinth: Designs and Meanings over 5,000 Years".
In addition to that exhaustive repertory of sources, the book includes several sections where Kern evaluates the present knowledge and ideas concerning the history and meanings of the labyrinth. In particular, he reviews in a detailed and critical way the recent theories and hypotheses concerning both general historical developments and particular documents. This, to a certain point, and at a relatively low cost, permits one to get acquainted with that specialized and often inaccessible literature (inaccessible both because of language barriers and scarcity of available copies).
All presently known documents and manuscripts seem to be included in the English edition of Kern's book. That is, with one significant exception that I know of: a drawing of the Sens floor labyrinth made before its destruction, discovered by Craig Wright in 1996 and published by him in his 2001 book. Kern discusses the Sens labyrinth under No 289 (German edition: No 285).
Kern's historical and documentary research has been an authority since its first publication in Italian in 1981. This is why it had to be published in English. However, Kern's opinions remain opinions. On certain points, he does not convince me (for example, on the relationship between the graphic motif of the Cretan labyrinth and certain ritual dance practices (p. 43-47): I rather think that these ritual dances should be considered as directly derived from the mythical labyrinth motif, whose form can neither be described nor represented. That derivation would be independent from that of the graphical motif, so that there can be no direct formal correspondance between the two derived motifs).
On other points, I disagree with him. For example, on what he calls the "christianization" of the labyrinth through the superimposition of the motif of the cross (p. 105-106): I think it is simply a case of borrowing from the Roman labyrinth the partition of the labyrinth into four quadrants. Likewise, Kern considers Eric of Auxerre's labyrinth as an "immature prototype" of the Chartres labyrinth (p. 114): I consider it as a sophisticated tentative to design a transformable labyrinth based on the Chartres labyrinth.
Kern's objective and competence were not to renew the discourse on the labyrinth as a graphic motif, but to draw the inventory of known labyrinths, of documents representing them and of works about them. However, it seems to me that Kern's review of the literature about the graphic motif of the labyrinth is both complete and trustworthy.
Kern's book being as it is complete, unique and now easily accessible, it has become de facto the universal reference. I refer to it systematically for the purpose of identifying all labyrinth documents and models. I use the illustration numbers from the English edition (it is not the same as in the German editions). (These references are not yet implemented on the present site, they will be soon).
Kern's is the only book that I can recommend absolutely. Its price, which is equivalent to that of three or four other books, is a rapidly recuperated investment, since it will precisely exempt from buying those books, and even many others.