There are a number of books about German New Guinea available in PDF format in the public domain. Here are some of them.
In a German Colony or four weeks in New Britain by B. Pullen-Burry. Report of an Englishman's visit to German New Guinea, published in 1909. (11 Mb)
Scented Isles and Coral Gardens by C.D. MacKellar. First-hand account of travel in the Torres Straits, German New Guinea and the Dutch East Indes, published in 1912. (17 Mb)
The Western Pacific and New Guinea: Notes on the natives, Christian and cannibal, with some account of the old labour tradeby Hugh Hastings Romilly. The title says it all, really. Published 1886. (11 Mb)
Adventures in New Guinea: The narrative of Louis Trégance by A French Sailor. A wholly fictional account of "Nine years in captivity among the Orangwoks, a tribe in the interior of New Guinea". Published 1894. (11 Mb)
Maps of German New Guinea:
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