Это та самая книга, которую подарил мне друг.

Это here means this is or it is. It is not the feminine adjective эта.

So:

• Это та самая книга... = This is that very book...
• Эта книга... = This book...

A useful way to think about it:

• это often introduces or identifies something
• этот / эта / это / эти are demonstrative adjectives that agree with a noun

In this sentence, Это is pointing out or identifying the book: This is...

Та самая is a very common emphatic phrase meaning something like:

• that very
• the very one
• that exact

So та самая книга means:

• that very book
• the exact book we were talking about
• the one in question

It adds emphasis and often suggests that the listener already knows which thing is meant.

Compare:

• та книга = that book
• та самая книга = that very book / that exact book

Here самый is not the pronoun meaning self. It is the emphatic word meaning very / exact / the very.

It changes to match книга, which is feminine singular nominative:

• masculine: самый
• feminine: самая
• neuter: самое
• plural: самые

So:

• та самая книга
• тот самый человек
• то самое место

This is a fixed and very common pattern in Russian.

Которую is the relative pronoun which / that, and it refers back to книга.

The base form is который, but it changes form depending on gender, number, and case.

Here it is:

• feminine singular, because it refers to книга
• accusative case, because it is the direct object of подарил

So the structure is:

• книга = the book
• которую подарил мне друг = which my friend gave me

In other words, inside the relative clause, the book is the thing that was given, so it must be in the accusative.

Because of the verb подарил.

The verb подарить means to give as a gift / to present. The thing being given is the direct object, and direct objects are often in the accusative.

Inside the clause:

• друг = the subject, the giver
• мне = the indirect object, the recipient
• которую = the thing given

So the hidden structure is basically:

• Друг подарил мне книгу
• Это та самая книга, которую подарил мне друг

Since книга would become книгу in a normal direct-object form, the matching relative pronoun is которую.

Because in Russian, relative clauses are separated by commas.

So:

• Это та самая книга, которую подарил мне друг.

The part которую подарил мне друг describes книга, so it is set off with a comma.

This is normal Russian punctuation with clauses introduced by который.

Russian word order is flexible, and the end of the clause often gets emphasis or contains newer information.

Here:

• которую подарил мне друг

is perfectly natural and means which my friend gave me.

Russian often places known information earlier and newer or more important information later. Ending with друг can make the giver sound like the final piece of information.

You could also say:

• Это та самая книга, которую мне подарил друг.

That is also natural. The meaning stays basically the same, but the emphasis shifts slightly.

Because мне means to me, and with подарить, the recipient is normally in the dative.

Compare:

• друг подарил мне книгу = my friend gave me a book
• мне = dative of я

Some common dative forms:

• я → мне
• ты → тебе
• он / оно → ему
• она → ей
• мы → нам
• они → им

So in this sentence, мне marks the person who received the book.

Because it agrees in gender and number with the subject in the past tense.

The subject is друг, which is masculine singular, so the past tense verb is:

• подарил = masculine singular past

Compare:

• друг подарил = a male friend gave
• подруга подарила = a female friend gave
• друзья подарили = friends gave

So the ending tells you something about the subject.

No. The grammar clearly shows that друг is the one who gave the book, not the one who received it.

Here is how the roles work:

• друг is nominative, so he is the subject
• мне is dative, so I am the recipient
• которую is accusative, so it is the thing given

So the only natural reading is:

• the book that my friend gave me

If you wanted the friend that I gave the book to, the structure would be completely different.

By itself, друг usually means friend.

In some contexts it can refer to a romantic partner, but that is not the default meaning. If someone specifically meant boyfriend, Russian more often uses:

• парень
• молодой человек

So in this sentence, a learner should understand друг as friend unless context suggests otherwise.

Yes. That would be completely grammatical.

• Это та книга, которую подарил мне друг. = This is that book which my friend gave me.

Adding самая makes it more emphatic:

• Это та самая книга... = This is that very book...

So самая is not required for grammar; it adds emphasis and specificity.

Yes. In English, both are possible depending on style:

• This is that very book which my friend gave me.
• This is that very book that my friend gave me.

In fact, in natural English, that is often more common here. Russian которую can correspond to either which or that in English.

A very helpful way to understand it is to break it into two simpler parts:

1. Это та самая книга.
   = This is that very book.

2. Друг подарил мне книгу.
   = My friend gave me a book.

Then Russian combines them by replacing книгу with the relative pronoun:

• Это та самая книга, которую подарил мне друг.

That is a useful model for building your own sentences with который.