Emmett Fields'
Bank of Wisdom offers collections of freethought
classics on CD-ROM:
http://www.bankofwisdom.com/
Among these are several works by and publications of Haldeman-Julius.
For a listing of works included, see:
http://bankofwisdom.com/library/
This may not be of use to collectors, but those interested in using
this material irrespective of collectability, here you go.
I bought several more Little Blue Books, which I'll list when I get a
chance.