It would seem that knowledge - or, rather, technology - is a public good as it is nonrival, nonexcludable, and has positive externalities. The New Growth Theory (theory of endogenous technological change) emphasizes these "natural" qualities of technology.
The application of Intellectual Property Rights (IPR) alters the nature of technology from public to private good by introducing excludability, though not rivalry. Put more simply, technology is "expensive to produce and cheap to reproduce". By imposing licensing demands on consumers, it is made exclusive, though it still remains nonrivalrous (can be copied endlessly without being diminished).
Yet, even encumbered by IPR, technology is transformative. It converts some public goods into private ones and vice versa.
Consider highways - hitherto quintessential public goods. The introduction of advanced "on the fly" identification and billing (toll) systems reduced transaction costs so dramatically that privately-owned and operated highways are now common in many Western countries. This is an example of a public good gradually going private.
Books reify the converse trend - from private to public goods. Print books - undoubtedly a private good - are now available online free of charge for download. Online public domain books are a nonrivalrous, nonexcludable good with positive externalities - in other words, a pure public good.