I need an authentic, trusted source (preferably online) for a precise
count of the number of words (not number of pages) of the Harry Potter
books. Of special concern are the word counts for books 2 (Chamber of
Secrets) and 3 (Prisoner of Azkaban).

this is quite impossible to find, isn't it ?
or best way is to contact the editor

or, you may be interested with this, it will help you to approximate
your count using the font and number of pages :
http://www.writersservices.com/wps/p_word_count.htm

'guess i was wrong with my previous post, indeed, i found this :
http://www.trelease-on-reading.com/rah_chpt7_p2.html

but it's not exactly what you are looking for, because it is about
tome 4 and 5 (maybe you'd allready found this page and that's why you
on ly want tomes 2 & 3

good luck

This site lists Scholastic's official word count for each book:

http://www.hp-lexicon.org/index-2.html


Chamber of Secrets: 85,141 words
http://www.hp-lexicon.org/about/books/book_cs.html

Prisoner of Azkaban: 107,253 words
http://www.hp-lexicon.org/about/books/book_pa.html