Boken er fascinerende.

Because boken is the definite singular form of bok.

In Norwegian, you usually show the by adding an ending to the noun rather than putting a separate word in front of it.

• bok = book (dictionary form)
• en bok = a book
• boken = the book

So Boken er fascinerende refers to the book, not just a book or books in general.

Norwegian often puts the definite article at the end of the noun.

So instead of:

• the book

you normally get:

• boken

This ending works like the. That is a very common pattern in Norwegian.

A few more examples:

• en bil = a car
• bilen = the car
• et hus = a house
• huset = the house

Er means is / am / are, depending on context.

It is the present tense of the verb å være (to be).

Examples:

• Jeg er trøtt. = I am tired.
• Du er snill. = You are kind.
• Boken er fascinerende. = The book is fascinating.

Unlike English, Norwegian does not change the present-tense form depending on the subject. So er works with I, you, he, she, it, we, and they.

Here fascinerende is being used as a predicate adjective: it comes after er and describes the subject.

Also, adjectives ending in -ende usually keep the same form.

So you can say:

• Boken er fascinerende.
• Filmen er fascinerende.
• Huset er fascinerende.
• Bøkene er fascinerende.

The form fascinerende stays the same.

This is different from some other adjectives, which do change:

• Bilen er stor. = The car is big.
• Huset er stort. = The house is big.
• Bilene er store. = The cars are big.

So fascinerende is one of the easier ones.

In this sentence, it functions as an adjective.

Historically, forms like fascinerende come from a participle-like form, similar to English fascinating, but for a learner it is usually most helpful to treat it as an adjective here.

So in:

• Boken er fascinerende.

fascinerende describes boken, just like interesting, boring, or exciting would.

This is the normal Norwegian word order for a simple statement:

• subject + verb + complement

So:

• Boken = subject
• er = verb
• fascinerende = description of the subject

This is very similar to English:

• The book is fascinating.

Norwegian word order becomes more interesting in other sentence types, but this one is straightforward and natural.

Yes, in Bokmål, that is also possible.

The noun bok can often be treated either as:

• common gender: en bok → boken
• feminine: ei bok → boka

So both of these can be correct in Bokmål:

• Boken er fascinerende.
• Boka er fascinerende.

The sentence you were given uses the common-gender/masculine-style definite form boken, which is very common, especially in more conservative or neutral Bokmål.

You would normally say:

• En bok er fascinerende.

Here:

• en bok = a book
• boken = the book

So the difference is:

• En bok er fascinerende. = A book is fascinating.
• Boken er fascinerende. = The book is fascinating.

That would be:

• Bøkene er fascinerende.

Here the noun changes to the definite plural form:

• bok = book
• bøker = books
• bøkene = the books

The adjective stays the same:

• Bøkene er fascinerende.

A simple learner-friendly pronunciation is something like:

• BOO-ken

A few notes:

• The o is a long vowel, not like the o in hot
• The stress is on the first syllable: BO-ken
• The final -en is usually light and unstressed

If you are using IPA, a common pronunciation is approximately [ˈbuːkən].

A rough learner-friendly guide is:

• fa-shee-neh-REN-de

The stress is usually on ren.

A few helpful points:

• The sc here sounds like sh
• The final -de is not strongly stressed
• Try to keep the rhythm smooth: fa-sci-ne-REN-de

Exact pronunciation can vary a little by speaker, but that guide will get you close enough to be understood.