It's clear that AW Pink had read a good number of "brethren" writers, and regularly quoted from them.

Is anyone able to explain his connection to "brethren," whether he simply appreciated their books or had a stronger affiliation? 

There are copious details of Arthur Walkington Pink (1886-1952) at <https://en.wikipedia.org/wiki/Arthur_Pink> including his early connection with Harry Ironside and Arno Gaebelein.  He seems to have been what I sometimes think of as 'an ecclesiastical rolling stone'.  Timothy Stunt

Since asking I have come across this book by Iain Murray, that has a fair bit of info about AW Pink and his connections to Ironside and Moody Bible Institute:

https://archive.org/details/lifeofarthurwpin0000murr/mode/1up

Murray's book on AWP exists in two editions, and the later edition is better and fuller. AWP's letters (Banner of Truth published a very small selection, but there are at least three fuller selections, such as "Arthur W. Pink: Letters from Spartanburg 1917-1920") show him preaching with brethren preachers in the US in his fundamentalist days, and again preaching for brethren in Glasgow in his later years. In fact, his preaching for the brethren in Glasgow might have been among his very last opportunities for public preaching before his removal to the Hebrides and into ecclesiastical isolation. In his articles on sanctification, which were published as a book after his death, AWP states that he found brethren writers more helpful than Reformed writers in terms of that particular doctrine.