يقرأ صديقي الكتاب كاملا.

A fully vocalized version is:

يَقْرَأُ صَدِيقِي الْكِتَابَ كَامِلًا

A common transliteration is:

yaqraʾu ṣadīqī al-kitāba kāmilan

A few pronunciation notes:

• يقرأ contains a hamza: yaqraʾu
• ص is an emphatic s
• ال in الكتاب is pronounced al- here, because ك is a moon letter, so there is no assimilation

Because Arabic very often uses verb–subject–object word order, especially in straightforward statements.

So:

• يقرأ صديقي الكتاب = My friend reads the book

This is completely normal in MSA.

You can also say:

• صديقي يقرأ الكتاب

That version is also correct, but it puts more attention on my friend as the topic.

It can mean either one, depending on context.

In MSA, the imperfect verb form often covers both:

• he reads
• he is reading

So يقرأ صديقي الكتاب كاملا could mean:

• My friend reads the book completely
• My friend is reading the book completely

Context tells you which is meant.

Because the ending ـي is the attached pronoun meaning my.

So:

• صديق = friend
• صديقي = my friend

Arabic often attaches possessive pronouns directly to nouns instead of using a separate word like English my.

Because صديقي is already definite through possession.

In Arabic, a noun with a possessive ending like ـي is automatically definite:

• صديقي = my friend, not just a friend of mine

Also, when a noun is followed by an attached possessive pronoun, you do not add ال.

So:

• صديقي = correct
• الصديقي = not correct for my friend

صديقي is the subject of the verb, so it is nominative.

The basic idea is:

• صديقُ = friend (nominative form)
• صديقُ + ي = صديقي

When the possessive ـي is attached, the usual case ending is not clearly visible in the same way it is on a fully declined standalone noun. So the word is still functioning as nominative, even though you do not see a separate final ـُ in the normal written form.

Because it is the direct object of the verb يقرأ.

The thing being read is the book, so in full vocalization it becomes:

• الكتابَ

That final fatḥa marks the accusative case.

In ordinary everyday Arabic writing, short vowels are usually omitted, so learners often see just:

• الكتاب

But grammatically, in a fully vocalized sentence, it is الكتابَ.

كاملا literally means complete / whole / entirely, and here it is being used in an adverb-like way.

So it gives the sense of:

• completely
• in full
• the whole thing

Grammatically, it is in the accusative: كامِلًا

A useful point: this is not a regular adjective directly attached to الكتاب. If it were a normal attributive adjective, it would match in definiteness too, like:

• الكتاب الكامل = the complete book

But الكتاب كاملا means something more like:

• the book, completely
• the whole book
• the book in its entirety

Because here it is not simply modifying the noun as an ordinary descriptive adjective.

Compare:

• الكتاب الكامل = the complete book
  This is a normal noun + adjective combination, and both words are definite.

But in your sentence:

• الكتابَ كاملًا

كاملاً is being used in an accusative adverbial/state-like function, not as a basic descriptive adjective. That is why it appears indefinite and accusative.

In practice, learners usually understand it as meaning that the book is read completely / in full.

So the natural interpretation is:

• My friend reads the whole book
• My friend is reading the book completely

Because both صديقي and الكتاب are masculine singular, the form كاملا does not by itself make the reference perfectly obvious. The meaning usually makes it clear.

If you want a very clear way to say the whole book, Arabic often uses:

• يقرأ صديقي الكتابَ كلَّه

That is a very common and clear alternative.

Because the verb يقرأ already includes the meaning he reads / he is reading.

Arabic verbs usually show the subject through their form. Then, if you also mention the subject noun:

• يقرأ صديقي

the noun صديقي specifies who that he is.

So adding a separate pronoun like هو would usually be unnecessary here.