The club of both shooters and reloaders!
THE best primer on reloading, IMHO, is
The ABC's of Reloading published by Krause
www.krause.com if not available locally, or you could order it from any of the shooting supply houses. It covers the "what" and the "how" of reloading very completely. It is not a loading manual--You will need one of those too, at least. More manuals are better, as no one manual can cover all possible combinations of bullet and powder. Most experienced reloaders have several manuals, and check all of them when trying to develop a new recipie.
My personal favorite is Lyman's 48th edition manual. Lyman does not make powder or bullets, and therefore IMHO is more impartial. But I check other manuals too.
Any manual you use will have the reccommended OAL for a given cartridge/bullet. The OAL varies with the bullet. For starters, go with the reccommended OAL.
With your extremes in temperature, I'd look into Hodgdon's "Extreme" line of powders; they are made to be temperature-insensitive. With the .30-'06 you have a wide choice of proper powders and bullets--perhaps the widest choices of any cartridge.
The more research you do beforehand, the more informed will be your decisions. It seems at first that there is too much information rather than not enough, but it sorts itself out given time. There are no reloading masters, just students at different levels, so never fear asking a question.
Safety is always the paramount consideration--NEVER exceed the maximum loads given.
Again, welcome to the club!