I have this book, and I am similarly frustrated by it's lack of detail.
The little grey boxes scattered throughout the book tell you what the numbers and letters mean; there's such a box on p.130, for example (I've got the second edition). The numbers stand for the different kinds of notgeld that were issued eg. paper notes, metal coins, POW camp tokens, etc. and the letters stand for the approximate rarity/value of that class of notgeld from that city.
For example, the entry for Gummersbach (also on p.130) reads "1B 2B 6A 14B". Referring to the table, this means that in the name of Gummersbach, there are known to have been issued "municipal paper" (paper notgeld issued by the town council), "private paper" (paper notes issued by private businesses and indivuals who were based in that town), "municipal metal" (metal notgeld tokens issued by the town council) and "WWII issues" (paper notes issued during the WWII period). The metal tokens are rated "A", the commonest category, while the three types of paper are rated "B", the next scarcest.
What this catalogue doesn't attempt in any way is break down the categories in any way, or let you know how many different types of notgeld there were issued in these different categories, or even if there are any scarce varieties to look out for. It's extremely general, just a checklist of issuing cities, really, and all but useless for determining the actual rarity or value of a specific piece of notgeld.
Unsatisfactory, but I guess anything more comprehensive would have been a huge, multi-volume task. The Keller notgeld catalogues cited in Coffing run to 30 volumes, and even this series is not "complete"!
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis