I used to search for and purchase all three of the Haynes, Clymers and Chiltons books available for every vehicle we owned as one or two would always have missing or omitted info, or one would have a helpful diagragm that the other doesn't.
Being the only trained (and capable) wrench-turner in the family at this time, I'm 'in charge' of maintaining and repairing the 20-some vehicles in our "family fleet" so having the correct info when I need it is very important.
Here's a good place for Factory Service Manuals and other hard to find related Specification Books and Unit Repair Manuals-
http://www.faxonautoliterature.com/If they don't have it, they'll look for it and contact you when they find it.
I've purchased several factory manuals from this place through past years either because they were the only place that had them or they had the best price. Beware of DVD copies seen on ebay or amazon, some may be okay, but I've found that many are missing certain info or are so scattered in the files that you may spend more time looking than what your time is worth.
I recently got a full factory set for a '79 Lincoln Versailles from Faxon and then they also found the supplimental books to the full set as well for me, which are about impossible to find anymore but contain certain specs or info is not mentioned in the bigger books... so at times, ya' gotta have 'em. Especially for rare cars like this with limited production.
Having these books when selling a car also boosts the sales attraction since it's often considered an "added bonus" because the buyer either doesn't have to find the books themselves or it may assure them a bit more about the proper care and service of the vehicle if they are added into the sale price and included with the purchase. I've done this a few times and was able to get the price I wanted by including the Factory books we had for it.
I got all the Factory manuals along with the Factory Assembly ones too (approved copies) for my '67 Coupe from Faxon. So I have all the technical info I would ever want about my car for a ground-up restoration or just how to fix anything on it without guessing where it is to find the part, the procedure to access it, the specs to test it and/or how to replace it.
Forums are great for sharing experienced info or for getting answers on some odd problem, but Factory books (or approved copies) are very well worth the investment for having all the correct info right there in front of you.
I searched other few mustang forums for information but no one can't help me with that...