Another freebie I came across on Google Books that you can download in free (pdf format) is 'The Fleets Behind the Fleet', by William Macneile Dixon. The author was clearly passionate about the Merchant Navy, and had some interesting anecdotes and stories about the merchant vessels in general, as well as minesweepers, trawlers, and the like. One that caught my interest was the story of a merchant ship nearly dropping anchor on a u-boat resting on the bottom; the u-boat surfaced to see who had disturbed them, and one of the merchant mariners jumped aboard the surfacing u-boat, hammer in hand, and smashed the periscope! Now, *that's* chutspah.
I also just snagged "Defensively-armed Merchant Ships and Submarine Warfare" by Alexander Pearce Higgins. If folks are interested I will post a review once I have read it.
Any other great, free books you know of that are essential (or at least entertaining) reading on merchant ships/ DAMS in WWI?