Gentoo doesn't provide 8.24, at least, not anymore.
Code:
$ ls -l /usr/portage/app-text/calibre/
total 52
-rw-r--r-- 1 root root 28162 Dec 5 20:01 ChangeLog
-rw-r--r-- 1 root root 1424 Dec 5 20:01 Manifest
-rw-r--r-- 1 root root 5495 Nov 25 17:43 calibre-0.8.28.ebuild
-rw-r--r-- 1 root root 5495 Dec 5 19:49 calibre-0.8.29.ebuild
-rw-r--r-- 1 root root 340 Nov 15 17:01 metadata.xml
Both 8.28/29 detect my device properly. Maybe your Gentoo is not updated?