Learn Russian: Он имеет много книг.

You learned earlier that 'he has' is usually у него есть. While that is the most natural way to express possession in everyday Russian, the verb иметь (to have or possess) is also correct. Using имеет here makes the sentence sound a bit more formal or literary. In casual conversation, a Russian would almost always say У него много книг instead.

Why is the word for books spelled книг here?

After words expressing quantity like много (a lot or many), you must use the Genitive case. Since we are talking about multiple books, it takes the Genitive plural form. The dictionary form is книга (a feminine noun ending in -а), and its Genitive plural drops the ending entirely, leaving just книг.

If I wanted to add 'which are old' to this sentence, how would I use который?

You would add a comma and say: Он имеет много книг, которые старые. Even though книг is in the Genitive case in the first half of the sentence because of много, the word которые (which) acts as the subject of its own new clause. Therefore, которые takes the Nominative plural case to match the plural books.

What conjugation pattern does иметь follow?