Recensionen var inte lång, men den hjälpte mig att välja mellan två böcker.

Because recensionen is the definite singular form: the review.

In Swedish, the definite article is usually attached to the end of the noun:

• en recension = a review
• recensionen = the review

Since recension is an en-word, its definite ending is -en.

Yes. Recension is the standard word for a review of something like a book, film, play, or similar work.

So in this sentence, recensionen most naturally means the review of a book.

Var is the past tense of vara (to be).

• är = is
• var = was

So:

• Recensionen är inte lång = The review is not long
• Recensionen var inte lång = The review was not long

Because the adjective agrees with recensionen, which is a singular en-word.

After vara and similar verbs, Swedish adjectives still agree with the subject:

• en-word singular: lång
• ett-word singular: långt
• plural: långa

So:

• Recensionen var lång = correct
• Brevet var långt = because brev is an ett-word
• Böckerna var långa = plural

In a normal Swedish main clause, the negation inte usually comes after the finite verb.

So:

• Recensionen var inte lång

This is standard Swedish word order:

1. subject
2. finite verb
3. negation/adverb

Compare:

• Han kommer inte idag
• Boken är inte dyr

Den means it, and it refers back to recensionen.

Swedish uses:

• den for en-words
• det for ett-words

Since recension is an en-word, the correct pronoun is den.

So:

• recensionen → den
• brevet → det

This is a very common Swedish pattern:

hjälpa + someone + att + infinitive

It means help someone to do something.

So:

• hjälpte = helped
• mig = me
• att välja = to choose

Together:

• den hjälpte mig att välja = it helped me choose

Yes, often you can.

Both of these are possible:

• hjälpte mig att välja
• hjälpte mig välja

The version with att is very common and a little fuller. The version without att is also natural, especially in everyday speech.

Because mellan means between, and it is the natural choice when talking about a choice involving two distinct options.

• mellan två böcker = between two books

Bland means more like among, usually in a larger group.

So:

• välja mellan två böcker = choose between two books
• välja bland många böcker = choose among many books

Yes. Bok is an irregular noun in the plural.

Its forms are:

• en bok = a book
• boken = the book
• böcker = books
• böckerna = the books

The vowel changes from o to ö, which happens in some Swedish nouns.

Not really. In modern Swedish, a comma before men is often optional and many writers leave it out.

So both of these are possible:

• Recensionen var inte lång, men den hjälpte mig att välja mellan två böcker.
• Recensionen var inte lång men den hjälpte mig att välja mellan två böcker.

The comma can make the pause clearer, but it is not required here.

After men, you start a new main clause, so normal main-clause word order applies.

Here the second clause is:

• den hjälpte mig att välja mellan två böcker

That is:

1. subject: den
2. finite verb: hjälpte
3. rest of the sentence

So men does not create subordinate-clause word order. It simply connects two main clauses.