Nieuwenkamp wrote a great number of books that he always illustrated himself (see Useful links and publications). He wrote about everything that fascinated him.
He was also a trained lithographer. His major book: Bali and Lombok, the first standard work on indigenous Balinese art and culture, was printed and published entirely on board the De Zwerver. Later in 1910, Elsevier published a miniaturized version of this book, later reworked and reprinted in 1922. Het Handelsblad [Dutch newspaper] published the letters he wrote about his travels/wanderings around Bali.
Not a writer pur sang?
In his preface to Bali and Lombok, Nieuwenkamp stated that although he was not a writer, he simply had to point to all the unusual things that he came across travelling around Bali, simply because there was no one else to do it. Yet later he was repeatedly praised as a writer and, in 1906, he was appointed member of the Society of Dutch Literature.
Nieuwenkamp wrote almost daily, mostly in the evenings, after all the other work was done. His writings were often quite humorous.