It was mentioned that the book Smalltalk The Language And Its Implementation is out of print. There are other technical books that are out of print which are equally useful both for a historical snapshot and for insights into how people were thinking at the time. I'm still quite fond of my copies of Starting Forth and Thinking Forth, both of which are out of print (or seem to be).
Sometimes the author of the book kindly converts the books to an electronic form and republishes them online. Philip J. Koopman, Jr. wrote Stack Computers: The New Wave which is an important book for anyone interested in the design and history of stack-based computing. It's available at www.ece.cmu.edu 
for those that enjoy such.
And there are other books that are important. I don't remember the title, but I once had read a book about the UCSD p-System's design and implementation that is no longer in print. It was a fair book, but would be interesting to toss into the hands of Java Programmers who think the Java Virtual Machine and the idea of "Write Once Run Anywhere" is something new.
So much is lost when these books aren't available. There has to be some way to preserve this history and make it available to others. Until recently, I lived across the street from a local historical society that wanted to preserve local building and monuments of local importance. Is there anything like this for Computer Science and other technical references? Are there people who (like myself) see the value in such a thing?
Well, yes. I imagine the problem is dealing with the copyright issues. You can't republish something like the UCSD book without permission from its authors, and discovering who they were and contacting them can be a job in itself. -- Dave Harris
Some booksellers have released their out-of-print books on the web. OReillyAndAssociates has done it with their Open Books Project.
This would be good option for those technical books that have a short shelf life (Books about Castanet, Hyper Talk, Open Doc, and ).
In many cases the author retains copyright of the work when the publisher's contract is over. Many authors do release their books on-line, sometimes even when it's not out of print (Bruce Eckel for one). In those cases, it's probably up to the author.
Project Gutenberg is the place to get classic literature that are out of copyright.
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