Arranged index on a couple of pages: it may be at back of book (8)
I believe the answer is:
appendix
'it may be at back of book' is the definition.
The definition suggests a singular noun which matches the answer.
'arranged index on a couple of pages' is the wordplay.
'arranged' is an anagram indicator.
'on' says to put letters next to each other.
'couple of' says to double the letters.
'pages' becomes 'p'.
'p' duplicated is 'pp'.
'index' is an anagram of 'endix'.
'a'+'pp'='app'
'endix' after 'app' is 'APPENDIX'.
Can you help me to learn more?
(Other definitions for appendix that I've seen before include "Supplement at the end of a book" , "It contains supplementary material" , "It comes last" , "Reference" , "Addendum - vestigial pouch in human intestines" .)