Does objective knowledge mean perfect knowledge?

[Consul]

What is your criteria for the objectivity of knowledge?

Objective knowledge corresponds, precisely and mind-independently, with that which actually is.

Since knowledge entails truth, i.e. "conformity with fact, agreement with reality" (OED), all knowledge is per se objective in this sense. "False knowledge" is a contradiction in terms—as opposed to "false belief/judgment/opinion/conviction". You can falsely believe to know that p, but you cannot falsely know that p.

[Consul]

You can falsely believe to know that p, but you cannot falsely know that p.

You can also be falsely certain that p.

[Consul]

Also note that Karl Popper has a special concept of objective knowledge as "abstract" knowledge which doesn't depend on and isn't part of (the minds of) knowing subjects. This objective knowledge is impersonal in the sense that it doesn't belong to "the personal spirit" (Nicolai Hartmann's term) of an individual person.

"Popper develops the notion of objectivity further in a novel but controversial way by seeking to free it completely from all psychological constraints. What is central to epistemology, he reaffirms, is the concept of objectivity, which he seeks to show requires neither the notions of subjective mental states nor even that of a subject “possessing” knowledge: knowledge in its full objective sense, he argues, is knowledge “without a knowing subject”."

Karl Popper > Objective Knowledge and The Three Worlds Ontology

That difference is better denoted with the terms "knowledge" and "information." There may be unknown information, but there is no knowledge without a knower.

If knowledge is defined as a mental state, then you're right, of course; and the word "information" does sound less psychological than "knowledge". I agree with you that it seems more appropriate to say that books contain true information rather than knowledge, especially as books themselves know nothing—as opposed to the people reading them.

On the other hand, given Nicolai Hartmann's useful distinction between the personal or subjective spirit (individual minds and their respective contents), the objective spirit (the total intersubjective Geistesleben (spiritual life) of society with its institutional or organizational manifestations such as language, science, art, philosophy, law, and morality), and the objectified spirit (externalized, "materialized" spirit), the true information contained in physical texts can be called objectified knowledge that exists in abstraction from and thus independent of psychological knowers. More precisely, objectified knowledge depends for its origin and objectification (material externalization) on subjective knowledge (knowledge as a kind of mental state of subjects) and knowing subjects; but once it is objectified (materially externalized), it no longer depends for its existence on knowing subjects.

[Pattern-chaser]

If the adjective "objective" in the phrase "objective knowledge" is not redundant, then there must be some contrast between objective knowledge and non-objective knowledge; but what is non-objective or subjective knowledge then?

This conversation can go in many directions, and often does! But I will comment that it was never my intention to offer Objective and subjective knowledge as binary opposites, with no other options available. That which is non-objective is not necessarily 'subjective', in my view.