Learn Hardcore Russian: Мой спокойный друг читает книгу дома. - My calm friend is reading a book at home.

In your sentence, we don’t need “is” because читаeт already provides the verb:
Мой спокойный друг читает книгу дома.
literally: My calm friend reads a book at home.

So you never put a separate word for “is / am / are” in simple present statements like this. It appears in the past and future (e.g. был, будет), but not in the present.

Why is it мой спокойный друг, and not моя спокойный друг or мой спокойная друг?

Because друг is a masculine noun, and Russian makes other words agree in gender, number, and case with the noun.

друг = masculine, singular, nominative (subject of the sentence)
The possessive мой = masculine, singular, nominative (“my” for masculine nouns)
The adjective спокойный = masculine, singular, nominative

So the “all masculine, singular, nominative” pattern is:

мой друг – my friend (male)
мой спокойный друг – my calm friend (male)

If the noun changed gender, мой would also change:

моя подруга – my (female) friend
моё окно – my window (neuter)
мои друзья – my friends (plural)

What does the ending -ый in спокойный mean?

The ending -ый shows that спокойный is:

an adjective
in masculine
singular
nominative case

It agrees with друг, which is also masculine singular nominative.

Other common adjective endings in nominative:

Masculine: -ый / -ий / -ой (e.g. новый, синий, большой)
Feminine: -ая / -яя (e.g. новая, синяя)
Neuter: -ое / -ее (e.g. новое, синее)
Plural: -ые / -ие (e.g. новые, синие)

So you get pairs like:

спокойный человек – calm person (masc.)
спокойная женщина – calm woman (fem.)
спокойное море – calm sea (neut.)
спокойные люди – calm people (pl.)

What exactly does спокойный mean? Is it the same as “quiet”?

Спокойный primarily means:

calm, not nervous, even-tempered (about a person’s character or state)
peaceful, not turbulent (about a situation, sea, weather, etc.)

It can overlap with quiet, but it’s more about inner calm and lack of disturbance than just low volume.

Compare:

спокойный друг – a calm, even‑tempered friend
тихий друг – a quiet (soft‑spoken, not loud) friend

So in your sentence, спокойный друг suggests a calm-natured friend rather than just someone who speaks quietly.

Why is the verb читает used here, and not another form like читаю or читают?

The infinitive is читать = “to read.”
Читает is the 3rd person singular present tense form:

я читаю – I read / am reading
ты читаешь – you read (singular, informal)
он / она / оно читает – he / she / it reads / is reading
мы читаем – we read
вы читаете – you read (plural / formal)
они читают – they read

Your subject is друг (he), so you must use читает:

Мой спокойный друг читает… – My calm friend reads / is reading…

Russian doesn’t distinguish “reads” vs “is reading” in form; читает can mean both, depending on context.

Why is it книгу and not книга?

Книга is a feminine noun meaning “book.”

книга – nominative case (used for the subject)
книгу – accusative singular (used for a direct object)

In your sentence, “book” is the thing being read, so it’s a direct object, which takes the accusative:

Книга лежит на столе. – The book is lying on the table. (книга is the subject.)
Друг читает книгу. – The friend is reading a book. (книгу is the object.)

So читает что? книгу.
That’s why you see книгу.

Does книгу mean “a book” or “the book”? Where is the article?

Russian does not have articles (no “a / an / the”).

книга / книгу can mean “a book” or “the book”, depending only on context.

So читает книгу could be understood as:

is reading a book
is reading the book

In early learning, you usually choose whichever sounds more natural in English given the situation. Grammatically, Russian doesn’t mark that difference.

Why is it дома and not дом or в доме?

All three exist, but they have different uses:

дом (nominative) – “house, home” as a subject or simple noun:
- Дом большой. – The house is big.
дома (accent до́ма) – an adverb‑like form meaning “at home”:
- Я дома. – I am at home.
- Друг читает дома. – The friend is reading at home.
в доме – literally “in the house”:
- Он читает в доме. – He is reading in the house (more literal, inside some house; can sound slightly more concrete or specific than just “at home”).

In your sentence, дома is the natural way to say “at home”.

Where is the stress in each word, and how do you roughly pronounce the sentence?

Stresses (marked with an accent for clarity):

Мо́й – MOY
споко́йный – spa‑KOYN‑nyy (stress on ко́й)
друг – druk (with rolled or tapped r; г is like English g)
чита́ет – chee‑TA‑yet (stress on та́)
кни́гу – KNEE‑goo (stress on кни́)
до́ма – DO‑ma (stress on до́)

A rough transliteration:
Мо́й споко́йный друг чита́ет кни́гу до́ма.
Moy spakóynyy druk chitáyet knígu dóma.

How flexible is the word order? Could I say Мой спокойный друг дома читает книгу?

Russian word order is more flexible than English because case endings show who does what. In neutral style, Subject – Verb – Object – (place/time) is common:

Мой спокойный друг читает книгу дома.

You can move parts around, often to change emphasis:

Мой спокойный друг дома читает книгу.
Slightly emphasizes дома (“at home is where he reads the book”).
Книгу читает мой спокойный друг дома.
Emphasis on книгу (“it’s the book that my calm friend is reading at home”).

For learners, stick to the neutral order first. Acceptable alternatives exist, but random word order can sound unnatural.

Why is друг used for “friend”? What if the friend is female?

Друг is a masculine noun meaning “friend” (male friend, or in general when gender is not specified).

If you specifically mean a female friend, you usually say:

подруга – female friend

With мой / моя:

мой друг – my (male) friend / my friend (gender not specified)
моя подруга – my (female) friend

So:

Мой спокойный друг читает книгу дома. – My calm (male) friend is reading a book at home.
Моя спокойная подруга читает книгу дома. – My calm (female) friend is reading a book at home.

Notice how мой → моя and спокойный → спокойная change to match the feminine noun подруга.

Is читает describing a repeated habit (“often reads”) or an action happening right now (“is reading”)?

In Russian, the same present‑tense form usually covers both meanings:

Он читает книгу.
- He reads a book. (habit, general)
- He is reading a book. (right now)

Context decides which English translation is better.

Your sentence with no extra context can be understood either as:

My calm friend reads a book at home. (habitual)
My calm friend is reading a book at home. (right now)

Russian doesn’t have a special present continuous form like “is reading.”