Since you asked for "best" I would off a couple of suggestions. If you had asked for a "great" or "prefect" one - would have not idea
Get a copy of all the different Assembly Manuals (Osborn Reproductions) and a set of Shop Manuals. The Assembly Manuals will have great exploded views while the Shop Manuals will have step by step instructions for installing and troubleshooting
Now IMHO be warned that neither is a restoration guide, so that is why (well one of the reasons) you have forums like this one. Also both of the printed resources include details that are not correct for your car and or plant. The shop manual also will use generic pictures and illustrations at times rather than Mustang specific. Both issues can cause challenges but if you follow up here with a question or two someone should be able to steer you around the rough spots.
Good luck
