I put up 16 metres of shelving four years ago and already there are piles of other books all over the floor. Given the prices paid for 2nd hand railway books, I believe my wife should see this as an investment which she can realise when I go past the final ticket barrier!
The Gelwood site has fantastic detail, but are the PRR diagrams sufficient for MSTS modelling? If so, then Steam Locomotives of the Baltimore & Ohio, An All-time Roster
by William Edson, ISBN 0-9632913-0-0 gives equivalent quality diagrams for the B&O.
There was a series of compilations from Locomotive Cyclopaedias
from between the wars; I have No.14, which has detailed drawings of various 4-8-4s. Nos.1, 2, 6 & 9 have further locos. I haven't seen these for a long time - best bet is to search on http://www.abebooks.co.uk
Finally there were a large number of Loco Profiles devoted to North American steam: No.2 - NYC Hudsons, 6 - Mallets, 14 - PRR Pacifcs, 16 - UPRR 4-12-2s, 20 - 4-8-4s, 24 - PRR Duplexii, 26 - Milwaukee Hiawathas, 28 - SPRR Cab-in-fronts, 31 - Lima super-power, 35 - CPR Selkirks. All these have plentiful illustrations, a beautiful colour drawing of the loco and sometimes line drawings. Again, try http://www.abebooks.co.uk
There are many good bookshops in the US, and often - if you can live with the slow delivery by surface mail - much cheaper than in the UK. I use http://www.karensbooks.com
but if you look in the back of Trains
or Classic Trains
magazines you will see plenty of others.
Good hunting! And if you don't make a model for MSTS, you can still dream and drool over what was the greatest steam spectacular on the planet.