Bókin er um ást.

Because bókin means the book, while bók means a book or just book.

Icelandic usually puts the definite article at the end of the noun instead of using a separate word like English the.

• bók = book
• bókin = the book

Here, -in is the definite article ending added to bók.

The base form is bók.

A learner will often want to strip off the article ending to find the dictionary form:

• bók = dictionary form
• bókin = bók
  • definite article

Also, bók is a feminine noun, which affects how it declines.

Er means is.

It is the 3rd person singular present tense of the verb vera (to be).

So:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is

In Bókin er um ást, er links the subject bókin with the phrase um ást.

In Icelandic, um is the normal preposition for about when talking about a topic or subject matter.

So:

• bók um ást = a book about love
• mynd um Ísland = a film about Iceland

Be aware that um can also mean other things in other contexts, such as around, but in this sentence it clearly means about.

Because this sentence is talking about love in general, not a specific love.

• ást = love
• ástin = the love

In English, we also often leave out the with abstract ideas:

• The book is about love
  not usually
• The book is about the love

If you said um ástina, that would usually suggest some specific love already known from context.

After um, Icelandic normally uses the accusative case.

So in um ást, ást is accusative. However, this can be confusing because the noun ást has the same form in nominative and accusative singular.

So even though the form does not change, the grammar still matters:

• nominative: ást
• accusative: ást

This is very common in Icelandic: sometimes the case changes without a visible change in the word.

Actually, this sentence is quite close to English word order:

• Bókin = The book
• er = is
• um ást = about love

So the structure is:

subject + verb + prepositional phrase

That is a very normal Icelandic sentence pattern. In more complex sentences, Icelandic word order can differ more from English, but this one is straightforward.

A simple learner-friendly guide would be:

• Bókin ≈ BOH-kin
• er ≈ ehr
• um ≈ um
• ást ≈ owst

A few useful vowel notes:

• ó is a long vowel, roughly like o in go, though not exactly the same as English.
• á is roughly like ow in cow.
• Icelandic stress usually falls on the first syllable, so BÓ-kin.

Yes, it is grammatical and natural.

It is a simple, clear way to say that the book’s topic is love.

That said, Icelandic also often uses fjalla um for books, films, and articles when talking about what they are about:

• Bókin er um ást. = The book is about love.
• Bókin fjallar um ást. = The book deals with / is about love.

Both are good, but fjallar um is especially common when describing content.

Ást is a feminine noun.

Yes, that matters in Icelandic because gender affects:

• article endings
• adjective agreement
• pronouns
• declension patterns

For example, if you wanted to say the love, you would say ástin, using a feminine definite ending.

Even when there is no adjective in this sentence, knowing the gender helps you learn the noun correctly from the start.