Nội dung của bưu kiện là sách.

Yes, nội dung often means content in the sense of information or subject matter, but in this sentence it means the contents of something.

So nội dung của bưu kiện means the contents of the parcel/package.

This can feel a little broader in Vietnamese than in English. In English, contents is often the most natural translation here.

Của is a very common word used to show possession or association, similar to of or sometimes 's in English.

So:

• nội dung = contents/content
• của bưu kiện = of the parcel

Together:

• nội dung của bưu kiện = the contents of the parcel

A very common pattern is:

• X của Y = Y's X / the X of Y

Examples:

• nhà của tôi = my house
• tên của anh ấy = his name
• màu của xe = the color of the car

Là is the verb that links two noun phrases, similar to is / are in sentences like A is B.

In this sentence:

• Nội dung của bưu kiện = subject
• là = is
• sách = books / book(s)

So the structure is:

• X là Y = X is Y

Here, là is needed because sách is a noun.

A useful comparison:

• With nouns, Vietnamese often uses là:
  
  • Anh ấy là bác sĩ. = He is a doctor.
• With adjectives, Vietnamese usually does not use là:
  
  • Cô ấy đẹp. = She is beautiful.

So là is there because the sentence is identifying what the contents are.

Vietnamese nouns usually do not have articles like a, an, or the, and they often do not change form for singular or plural either.

So sách can mean:

• book
• books
• the book
• the books

The exact meaning depends on context.

In this sentence, English will usually translate it as books or book(s) because contents of a parcel suggests one or more items inside.

If Vietnamese wants to be more specific, it can add other words:

• một quyển sách = one book
• những quyển sách = some books
• các quyển sách = books / the books

But in many everyday sentences, the bare noun sách is enough.

Vietnamese classifiers are common, but they are not always required.

For books, a common classifier is quyển or cuốn:

• một quyển sách = one book
• hai cuốn sách = two books

But when speaking more generally about the category of thing, Vietnamese often uses just the noun:

• Đây là sách.
• Nội dung của bưu kiện là sách.

So sách by itself is natural here because the sentence is stating what the parcel contains in a general way, not counting individual books.

No. That is exactly why nội dung is important.

• Bưu kiện là sách would sound like The parcel is a book, which does not make sense in normal context.
• Nội dung của bưu kiện là sách means The contents of the parcel are books.

So nội dung của tells us we are talking about what is inside the parcel, not what the parcel itself is.

Yes, bưu kiện is a standard word for a parcel or package, especially in postal or shipping contexts.

Some related words:

• bưu kiện = parcel, postal package
• gói hàng = package, parcel, shipment
• kiện hàng = package/cargo unit
• bưu phẩm = postal item, mail item

In many everyday situations, gói hàng is also very common.
But bưu kiện fits well when talking about something sent through the mail or delivery system.

Yes, it is grammatical and understandable. It sounds a bit formal or factual, like something you might see in a description, shipping note, customs declaration, or official statement.

In more everyday speech, Vietnamese speakers might also say things like:

• Trong bưu kiện có sách. = There are books in the parcel.
• Bưu kiện này đựng sách. = This parcel contains books.

So your sentence is correct, but it has a slightly more formal, definitional feel.

Not as a normal equivalent of this sentence.

Trong bưu kiện có sách is the natural way to say There are books in the parcel.

Why?

• trong = in/inside
• có = have/there is/there are

So:

• Trong bưu kiện có sách. = Inside the parcel, there are books.

Using là after trong bưu kiện would not be the normal structure for this meaning.

So keep these patterns separate:

• Nội dung của bưu kiện là sách.
• Trong bưu kiện có sách.

Both can express a similar idea, but the grammar is different.

A word-by-word breakdown looks like this:

• nội dung = contents/content
• của = of
• bưu kiện = parcel/package
• là = is/are
• sách = book/books

So the overall structure is:

[noun phrase] + của + [noun] + là + [noun]

More naturally:

The contents of the parcel are books.

This is a good example of Vietnamese putting the possessed thing first:

• nội dung của bưu kiện literally: contents of parcel

A rough guide for an English speaker:

• Nội: sounds a bit like noy, with a falling-glottal kind of tone in Northern Vietnamese
• dung: like zoong in the North, yoong or zoong depending on accent
• của: roughly koo-uh with a dipping tone
• bưu: roughly byoo
• kiện: roughly kee-en with a falling-glottal tone
• là: roughly lah with a low falling tone
• sách: roughly sack with a sharp rising/glottal tone

A very rough Northern-style approximation: Noy zoong koo-uh byoo kee-en lah sack

Important note: Vietnamese tones matter a lot, so any English-style spelling is only approximate. If you are learning pronunciation, it helps to listen to native audio and repeat whole chunks:

• nội dung
• của bưu kiện
• là sách

Sometimes, yes, depending on context.

Possible alternatives include:

• đồ bên trong bưu kiện = the things inside the parcel
• thứ ở trong bưu kiện = the thing(s) in the parcel
• bên trong bưu kiện có sách = inside the parcel there are books

But nội dung của bưu kiện is a neat, compact, and somewhat formal way to say the contents of the parcel.

So if you want a more official or written style, nội dung works well. If you want more conversational Vietnamese, other expressions may sound more natural.