Is there a Exifer documentation?
I want to know how I can read out exif information

Sorry, no. Exifier is something we inherited when the original developer abandoned it. We really cannot support outside of the context of Zenphoto.

We have actually the plain function documention online:
http://www.zenphoto.org/documentation/core/_exif---exif.php.html
and the related makers:
http://www.zenphoto.org/documentation/core/_exif---makers---nikon.php.html
http://www.zenphoto.org/documentation/core/_exif---makers---olympus.php.html
http://www.zenphoto.org/documentation/core/_exif---makers---panasonic.php.html
http://www.zenphoto.org/documentation/core/_exif---makers---sanyo.php.html
http://www.zenphoto.org/documentation/core/_exif---makers---fujifilm.php.html
http://www.zenphoto.org/documentation/core/_exif---makers---canon.php.html
http://www.zenphoto.org/documentation/core/_exif---makers---gps.php.html

But as my colleague said, we neither support nor test using it outside of Zenphoto (However, our "opponents" Gallery3 and Coppermine use it for exif reading as well, not sure if modified though). There are other tools available to read Exif. Your server might even have a an extension enabled, too.