It is better to see indexed addressing in terms of accessing arrays.
Say you have an array of 20 integers in memory.
(One integer = 4 bytes)
Now you can acces these in a loop, by using an index and the fixed multiplier of 4 (4 bytes is the size of one element of the array).
so 1st element: index = 0 in %edi, address with "start_array (,%edi,4)"
next element: increment %edi and use same statement ("start_array(,%edi,4)")
etc.
When you have an array of characters (1 byte) then your multiplier is 1.
This makes perfect sense to me, and is what I expected in the first place. Now that I read it again, the book does seem consistent with it---it's just described in a way I couldn't follow. Specifically, what I wasn't putting together well is that the multiplier specifies the length of an element (whether a record an array element), and the index register specifies "how many elements" from the address.
Anyway, got it now. Thanks to both of you.