writing device drivers for commercial hardware

As I'm new to linux kernel development I wonder how to write a 
device driver, say for a card reader, if you don't have some 
documentation about it.
How do you manage that? Searching for it in the web? I really don't 
know ... there's a small book with my card reader but you won't find 
a single line about technical stuff in it.
It's a (pretty simple and cheap) card reader connected to serial port.

Many thanks.

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Marc,
<[e-mail]>