There is also an interesting book by Richard E. Noble called "Number #1." It is the history etc. of the Highwaymen, not Cash et. al. but the folk group. Sadly, as of this date, only one of the originals is still living.
This one was published around 2015.
336-page hard cover book that looks at every song that The Seekers have recorded.
Lots of history, trivia and memorabilia.
Still available from Amazon for around $40-$50 new (less for used) or at a considerably higher price if you want one personally autographed from The Seekers' website.
I bought one for a good friend of ours as well as an avid Trio fan a few years ago.