Today there a number of eBook readers. What was the first device marketed as a device primarily for reading eBooks? When did it become available?
I think NuvoMedia's Rocket eBook was the first e-reading device. IIRC it was released in 1998. SoftBook also made a pre-1999 e-reader; it was the confluence of two competing technologies (plus Microsoft's software reader) that led to the formation of the Open eBook Foundation and the creation of a single, open e-book format.
That would be the Sony Librie, forerunner of the Sony Reader line. It was introduced in 2004 and originally available only in Japan.
The Sony Reader line was introduced in 2006 and was one of the first readers available in the US.
More information about various readers, dates they were introduced and capabilities can be found here (though the Librie is not listed).