By way of introduction, I've read most of the biographical material on Douglas as well as his writings. There have been six biographies written about him, and this is the best of the bunch. Here they are:
The first was by his friend and mentor, William Jackson Hooker, and was published soon after Douglas' untimely death under the title "A Brief Memoir of the Life of Mr. David Douglas, with Extracts from His Letters." This work is available free on the internet and I highly recommend it. It's brief, sympathetic, and hits the high points.
Perhaps because Douglas' journals went unpublished until 1914, and then were put out only in an expensive limited edition, he did not attract another biographer until 1947 when H.T. Hervey published "Douglas of the fir: a biography of David Douglas, botanist." This was an excellent work, and the later biographies repeat much material first presented by Hervey. It is long out of print and you will likely have to get it through a library.
The third Douglas biography is more of a curiosity than a work of reference; it contains much wild speculation that is unsupported or actually contradicted by the historical evidence, and overall seems to be an effort to turn Douglas' life into a pulp romance. Among other things it reveals that Douglas had a passionate affair with a dusky native lass on the shores of distant Gray's Harbor. You can read this and more in W. Morwood's Traveler in a vanished landscape;: The life and times of David Douglas.
Next up was DOUGLAS OF THE FORESTS. THE NORTH AMERICAN JOURNALS OF DAVID DOUGLAS by John Davies, which mostly consists of excerpts from Douglas' journals but also covers some biographical ground. It's a decent work, but mostly of interest because Douglas' own journals have only been published in rare and expensive limited editions (fortunately they can also be downloaded for free on the Internet).
After that we had David Douglas: Explorer and Botanist by Ann Lindsay, reissued a few years later under the title The Tree Collector: The Life and Explorations of David Douglas. Regrettably, it was not well fact-checked, and contains numerous false statements that betray the authors' general lack of familiarity with either botany or the geography of western North America, but which also contains some interesting facts which were not earlier published, and are revealed in the writings of Douglas' contemporaries.
The best came last, though: Jack Nisbet's "The Collector" easily displaces Hervey's work as the best Douglas biography, for it was prepared by a modern, rigorous historian with access to a breadth of material and interpretation that the earlier authors could not reach. Moreover, Nisbet has previously written a good number of books on the region's history and, apart from the synergistic effects on this research effort, it has allowed him to develop as a capable and engaging writer. It contains information not discovered by any prior writer, such as the news that Douglas had a son during his travels in the Pacific Northwest; it also does a far better job than any prior biography of placing Douglas within the society of his contemporaries both on the frontier and in the world of London's Linnean Society. Highly recommended.