I cant help much with the MoPac, Rock, or Frisco. Only book I've read on the MoPac was "The Rebirth of the Missouri Pacific, 1956-1983" by H. Craig Miner, which was a decent corporate history, but seemed a bit partial to the RR. I have not read much of anything on the Frisco...

For SP, there are a TON of very well done histories.
- Don Hofsomer's "Southern Pacific 1901-1985" is a very well done corporate history, which covers the RR up to the SPSF merger attempts.
- Any of Signor's books on SP divisions are very well done, and have good history, info on ops, maps, photos, etc.
- Phil Serpico has done a few books on SP lines also which are also very good though perhaps a bit drier reading.
- Ed Austin and Tom Dill's SP in Oregon book is top notch, with good history, tons of maps, good info on operations, etc.
- Tom Dill and Jack Bowden's Modoc Line book is also great, probably one of my favorites.
- David Myrick's series on RRs of Nevada and Eastern CA, and RR of Arizona are both top notch too. Tons of good history, less info on post construction/steam era ops. His book on NM RRs is good too but perhaps a little less in depth.
- For SP lines east, Texas & New Orleans by A.D. McLennan is supposed to be a well written book though I have yet to read my copy, so hard to comment on.
- There are plenty of other good books on SP branch lines and subsidiaries by various authors which are also quite good out there.

For this Hill Lines:
- SP&S book by Tom Dill and Ed Austin is a good one, like their SP in Oregon book.
- North Bank Road by John T. Gaertner is also a very good corporate history/general history of the road that fills in some of the holes in the above volume.
- There are two SP&S volumes by Walter R. Grande that are supposed to be good books but I've never seen either.
- For GN: The Great Northern Railway a History by Hidy, Hidy, Scott, and Hofsommer is supposed to be a good corporate history. I have a copy but have yet to read it. Also The Great Northern Railway: A Pictorial Study by Wood and Wood is also on the shelf waiting to be read. Looks good but more photos than text.
- Only good histories I've read on the NP are several books on lines in MT and ID by Bill Taylor. These were decent histories, on the building of the lines, but after completion, very little is covered.

For Santa Fe:
- Signor, Serpico, and Myrick mentioned under SP are all have good works on various lines.

Someone else will have to chip in for Rio Grande too.

Hope this helps a little,
~Matt