Going back to the original post, and the original code, wanted to point out the obvious mistakes.
First the user's selection is fetched into an array, then the intent is to cycle through that array processing each selected entry. Line 3 pulls out one at a time each of those array elements, and places it into a simple variable called "enrty" (note the spelling here). Line 8 through 14 is then trying to use that simple variable to get entry field values, but by mistake is using a function call "entry()" instead of that simple variable. (The function "entry()" returns an array of the all of the entries in the library.)
First of all, do not use a name for your simple variable that is just a misspelling of a function call. Gets confusing. I would suggest using the name "e" instead. Then in lines 8-14 use that simple variable.
Line 3:
var e = a[i];
Line 8-14:
var bk = e.field('USBC_Book');
var bpf = e.field('Back');
var pfs = e.field('Pic fs');
var pg = e.field('USBC_Page');
var numb = e.field('USBC_Number');
var add = e.field('USBC_Add');
var cani = e.field('CanIdentifier');
Last three lines:
e.set('pic FS',resfs);
var b = e.field('pic FS');
e.set("USBC Front", b)
(I recognize that "enrty" was probably a simple typing error, but that typing error did create a simple variable.)