Learn Turkish: Bu kitap Türkçe mi?

In Turkish, the "to be" verb (am, is, are) is often implied in simple present tense sentences. By putting the words together as Bu kitap Türkçe, you are already saying "This book is Turkish." The question particle mi simply turns that statement into a question.

Why is the question word mi written as a separate word?

The yes/no question particle is always written with a space before it. Even though it is written separately, it still acts like a suffix because it follows vowel harmony rules based on the last vowel of the word right before it!

Why do we use mi here instead of mı or mu?

This is due to vowel harmony. The word before it, Türkçe, ends with the vowel e. In Turkish, if the last vowel of a word is e or i, the question particle that follows it must be mi.

Why is Türkçe capitalized?

Just like in English, names of languages (like Türkçe for Turkish, or İngilizce for English) and nationalities are proper nouns in Turkish. Therefore, they must always start with a capital letter.