Książka jest niby niedostępna.

Here niby means something like supposedly, apparently, or so they say.

The important nuance is that niby often suggests some distance, doubt, irony, or mild skepticism. So the sentence does not just state that the book is unavailable; it can imply:

• that this is what people claim
• that the speaker is not fully convinced
• that the situation may be questionable or annoying

So niby is often less neutral than a plain factual statement.

Because it agrees with książka, which is a feminine singular noun.

In Polish, adjectives agree with the noun in gender, number, and case, even when they come after jest.

So:

• książka = feminine singular
• niedostępna = feminine singular form of niedostępny

Compare:

• ten film jest niedostępny — masculine
• ta książka jest niedostępna — feminine
• to miejsce jest niedostępne — neuter

It is in the nominative case, because it is the subject of the sentence.

The basic structure is:

• Książka — subject
• jest — verb
• niby niedostępna — predicate phrase describing the subject

So this is a normal nominative subject + być + adjective structure.

Yes. That is a very natural alternative.

Both sentences can mean roughly the same thing:

• Książka jest niedostępna — The book is unavailable.
• Książka nie jest dostępna — The book is not available.

The difference is mostly stylistic:

• niedostępna is a single adjective
• nie jest dostępna is a negated verb phrase

In many situations, both are fine.
If you want to keep the nuance of niby, you could also say:

• Książka niby nie jest dostępna.

Because in Polish, nie is normally written together with adjectives.

So:

• niedostępna — correct
• nie dostępna — normally incorrect

This is the standard rule for adjectives, for example:

• nieładny
• niemożliwy
• niepotrzebny

So niedostępna is one word.

In a normal full sentence, yes.

Polish usually keeps the present-tense form of być in sentences like this:

• Książka jest niedostępna.

You may sometimes see or hear omission in:

• headlines
• notes
• very colloquial speech
• dramatic or elliptical style

For example:

• Książka niby niedostępna.

But that sounds shortened, not like the most standard full sentence. For learners, it is best to keep jest.

Yes. Polish word order is flexible, but changing it changes the focus or emphasis.

Possible variants include:

• Książka jest niby niedostępna.
• Książka niby jest niedostępna.
• Niby książka jest niedostępna.

These are all possible, but they do not feel exactly identical.

The original version places niby close to niedostępna, which makes the doubt or irony feel tied especially to the idea of being unavailable.

A learner should remember:

• Polish word order is flexible
• but not random
• moving niby can subtly change what sounds emphasized or questioned

These words can overlap, but they are not the same.

• podobno = apparently, I’ve heard that
  Usually more neutral.

• rzekomo = allegedly, supposedly
  More formal, often stronger in expressing doubt.

• niby = supposedly, so-called, sometimes kind of
  Often more colloquial and can sound ironic, skeptical, or slightly dismissive.

So in this sentence:

• Książka jest niby niedostępna.

the speaker may be suggesting that the book is said to be unavailable, but they are not fully buying that explanation.

It usually sounds at least a little skeptical.

Without niby, the sentence is a straightforward statement:

• Książka jest niedostępna.

With niby, the speaker often sounds like they are saying:

• The book is supposedly unavailable
• The book is “unavailable,” allegedly
• They say it’s unavailable, but I’m not sure I believe that

So yes, niby often adds a tone of doubt, irony, or mild criticism.