Could you try the following?
Edit modules.book_format.php. At line 42, you should see the following function. Add the line 'header_remove(); where indicated.
Code:
function do_work($smarty, $db) {
global $config, $mimetypes;
$path = $this->path;
$pathinfo = pathinfo($path);
===> header_remove();
$mt = NULL;
With that line of code in, I get the response header set:
Code:
HTTP/1.1 200 OK
Date: Mon, 13 Dec 2010 20:51:54 GMT
Server: Apache/2.2.17 (Win32) PHP/5.3.3
Content-Disposition: attachment; filename=1632___Flint__Eric.epub
Keep-Alive: timeout=5, max=96
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: application/epub+zip
All caching headers are removed. The result looks a lot like the one that works.