When is a scratch an issue?

Nope, there is no book. My conclusions are based solely on experience. And if you want to learn how to evaluate coins then you need to get that same experience.

Ken Bressett said it best -

"Grading is really very simple. All you need is four things :
1 - a good magnifying glass
2 - a good light
3 - a good memory, and
4 - 20 years of experience"

There are no shortcuts, there is no fast and easy way. There is only a whole lot of work, effort and time. Nobody can learn to do this in a hurry.

If you want to invest then there are two things that you need to do.

First and foremost - find a dealer that you can trust. He has already done all the work and spent all the time. In the beginning you HAVE to lean and depend on HIS knowledge and experience or you will lose your shorts.

Second - do the work and spend your own time as you go along. This will only help your dealer by guiding him in the direction that you want to go.

And there is a third thing actually - pray for luck. Because you are going to need it. The coin market can turn on a whim and no amount of experience or knowledge can overcome that.

My favorite quote of all time is this -

In the early 14th century the Abbot of Tournai, Gilles Li Muisis, made a rather astute comment. En monnoies est li cose moult obscure Elles vont haut et bas, se ne set-on que faire Quand on guide wagnier, on troeve le contraire.

Translation - "Coins are the most obscure things. Their value rises and falls, and one does not know what to do. When one thinks that he has gained, he finds the contrary - that he has lost".

Now that is as true today as it was over 700 years ago. And it will always be true. You would do well to not forget it.