Thanks, John, for this really interesting account of the IRCA's history.
As you rightly say good accounts of the railway and its locos are few and far between. One interesting book is "Guatemala Railways and Industry - Development and Decline" by Peter English, published by De Archaeologische Pers in the Netherlands in 1983. It has some errors, e.g., describing the Mallets as having come from Colombia, but is a good read all the same. It's been out of print for years but second hand copies are around at a reasonable price, e.g., at Amazon and AbeBooks. Another interesting book is "Development of Railroads in Guatemala and El Salvador, 1849-1929" published by The Mellen Press in 2001. As its title suggests it doesn't contain anything later than the 1920's but it's notable for being illustrated by many of Robert W. Richardson's photos from the 1960's which are, of course, uniformly excellent! It's still in print and used to have an affordable price but is now expensive - at least compared to most railway books.
Gerald Best's article is in the R&LHS's Bulletin no. 104 for April 1961 and of course is superb, as are his many photos. It occupies about two thirds of the magazine. The R&LHS still had copies in stock when I bought mine from them in April 2011, exactly 50 years later (!) but it's no longer listed on their website. Again second hand copies often come up.
Best wishes,
James