The book uses "indexing" to mean something other than what my local puzzling tradition calls "indexing".
…you might have wondered: OK, what does a Geocache Puzzler mean when they say "indexing"? (Seeing as how a geocacher asked me the converse question about puzzle hunters, it seems like a question you might have.) Roughly: Given a thingy which is the Nth item in some list, use the number N. (In geocache puzzling, you're often trying really hard to get numbers instead of a passphrase; you're probably trying to get some lat/long coordinates.) So if you're solving a puzzle geocache in San Francisco and staring at the phrase
Rubidium, Rhodium, Fluorine, Erbium
Hydrogen, Titanium, Cobalt, Carbon, Zirconium
…and someone told you "Use indexing!", then instead of trying to pull letters from words, you'd try looking at those elements' places in the list of elements, their atomic numbers: 37 45 9 68 / 1 22 27 6 40. And, as an experienced geocacher, you'd look at that and say "Of course those are lat/long coordinates for a place around here, just missing some punctuation."
You could combine these meanings of "indexing". If a geocacher and a BANGer saw
Helium, Boron, Lithium, Nitrogen, Carbon, Lithium, Helium...then the geocacher might use their meaning of "indexing" to get
Helium 2, Boron 5, Lithium 3, Nitrogen 7, Carbon 6, Lithium 3, Helium 2and point out discouragedly that these weren't making coordinates. Then the BANGer could use the other meaning of "indexing" to say
Helium 2=E, Boron 5=N, Lithium 3=T, Nitrogen 7=E, Carbon 6=N, Lithium 3=T, Helium 2=E…and together these people from two puzzling traditions could achieve ENTENTE.
