Hey, I've read the book. Here's how the "standard" arrangement of keys evolved on a 15 bar harp.

Carl Gutter's Chord Zither was strung with a C scale – no flats or sharps - and had three buttons so you could play in C (the people's key)...
F - G7 – C

Then, to sell more harps, they added a Bb (A#) string and two buttons, and, voilas, you could also play in F...
Bb – C7 – F – G7 – C
... using the same fingering you learned for C.

Then the first abomination. A few other strings and chords were added, because three chord songs get boring, but they were just tacked on the end...
Gm – A7 – Dm - Bb – C7 – F – G7 – C

... until some genius realized they would be easier to reach if placed on a second row ...
Gm – A7 - Dm
    Bb – C7 – F – G7 – C

... which left room for more chords, also useful for playing in C or F. It also becomes possible to play in G on the east end of the row of buttons, as long as you stick to 3 chord songs.
Gm – A7 – Dm – E7 – Am - D7
Bb – C7 – F – G7 – C – G

Next they extended the bottom row so you can play in Bb, a useful key for accompanying singers, and added a D chord to the top row.
   D - Gm – A7 – Dm – E7 – Am - D7
Eb – F7 - Bb – C7 – F – G7 – C – G
This is the "standard" arrangement. You can play pretty easily in C, F, or Bb, play in G as long as you don't need an Em, and play in D if you have exceptionally large or agile hands and aren't fond of Bm. In addition to the A# string, you also need C#, D#,F# and G# strings to make all them chords.


There's also a 21 chord arrangement that is laid out quite logically so you can play in Bb, F, C, G, D or A using the same fingering. Notice that I, IV and V are arranged in a triangle, kinda like the key of G on a 15 bar harp.
Eb – Bb -   F -   C -    G -    D -   A
    F7 – C7 –   G7 – D7 – A7 – E7 – B7
       Ab – Bb7 – Cm – Gm – Dm – Am – Em
Some folks don't like the big bar holder on a 21 bar harp. It does take up a lot of real estate that could be usefully used for strumming.

There are Oscar Schmidt harps with other arrangements. The Festival harps are set up to play in either F, C and G; or in G, D and A. The Wildwood Flower is diatonic harp in D.

For some real fun buy a bunch of 15 bar harps and convert them to diatonic instruments. Re-felting the bars to make different chords is easy once you figured out what notes to tune the strings to. You get the arrangement of chords that makes sense to you, and once you learn a tune in one key you can play it in any key for which you have a harp. The book (did I mention that there's a book) has lots of suggestions for how to arrange the keys.
- Phil