NORM - Not to get too far off topic, but there really isn't one book that best describes the way IH managed their business. There's some great articles in Red Power Magazine every issue about IH history. Several of the suthors have access to the IH Corp. Archives at the WI State Historical Society Bldg in Madison, WI. A search should get you WEEKS of interesting reading. Think Guy Fay said there were four FULL semi-trailer loads of historcal documents & pictures given to the WSHS.
Also joining IHCC, IH Collectors Club, and getting their quarterly news letter, Harvester Highlights, is great too.
The book TOM H posted about, "A Corporate Tragety" by Barbara Marsh is a good start.
To answer your question about IH's demise, I think Door #4 is the best answer, All of the above. IH had a long history of an adversarial relationship with labor unions. I wouldn't say IH's Management was "Bad" (I was part of that management!) but their marketing group always kept forcing them into niche markets with low volume sales but high development costs which tied up design & engineering resources that should have been used to develope newer designs of the things they sold Millions of instead of hundereds of. Marketing also rushed engineering to release not sufficiently proven designs to market which caused recalls and loss of loyal customers. IMHO, post WW II they concentrated on trying to build small tractors, CUB, A/B/C to replace horses, a very small market once all the horses were replaced, and totally MISSED the need for improving and building larger row crop tractors for the midwest. Most of the competition saw this need and were able to get a jump on IH in the late 50's & 60's. FARMALL was building 325 FARMALL M's per DAY in 1950/'51 and in '52 sent parts to LVL to build an additional 50 Super M's plus their own 325/day, plus FARMALL was building H's & Super H's, but by the late 50's all those tractors were too small. IH ended up turning up the engine speed on the 560 Standard tractor and adding planetary reduction units to the ends of the rear axles to create the 660 International to make a five-bottom plow tractor. It wasn't until 1963 with the 806 that IH made a real 5-bottom tractor.
When IH put all their resources to something, great things happened. The FARMALL H/M, Cub Cadets, Axial Flow combines, plateless corn planters, most of their plows & discs, the 806, 1206, almost ALL the 56-series of tractors, most of their bigger trucks & semi-tractors. They just wasted too much time chasing small volume markets with little if any profit potential.