In Genki II 3rd edition, it looks like it’s chapter 15, section 4 “Using Sentences to Qualify Nouns”.
In the phase おもしろい本, the い-adjective おもしろい qualifies the noun 本 and tells us what kind of book it is. You can also use sentences to qualify nouns. The sentences that are used as qualifiers of nouns are shown in the boxes (in bold) below.
- 昨日買った 本 - the book that I bought yesterday
- 彼がくれた 本 - the book my boyfriend gave me
- つくえの上ある 本 - the book that is on the table
- 日本で買えない 本 - the book that you can’t buy in Japan
Qualifier sentences in these examples tell us what kind of book we are talking about, just like adjectives. The verbs used in such qualifier sentences are in the short forms, either in the present (as in examples 3 and 4) or the past tense (1 and 2), and either in the affirmative (1-3) or the negative (4). When the subject of the verb— that is to say, the person performing the activity— appears inside a qualifier sentence, as in example 2 above, it is accompanied by the particle が and not は.
A “qualifier sentence + noun” combination is just one big noun phrase. You can put it anywhere in a sentence that has a noun.