Learn Russian: Учебник новый и интересный.

Because учебник (textbook) is the subject of the sentence, not the direct object. The accusative case is for the thing being acted upon (like if you said 'I read the textbook'). Here, the textbook is just being described, so it stays in the nominative case.

If the textbook were the direct object, how would these words change?

They actually wouldn't change at all! For masculine inanimate nouns like учебник, the accusative case looks exactly the same as the nominative case. The adjectives новый and интересный also keep these exact same forms when they describe a masculine inanimate direct object.

Where is the word for 'is' in this Russian sentence?

Russian almost always skips the present tense of the verb 'to be' (am, is, are). To say 'the textbook is new,' you simply put the subject (учебник) right next to its describing words (новый и интересный) without a linking verb in between.

Why do both новый and интересный end in -ый?

Adjectives always have to match the gender, number, and case of the noun they describe. Since учебник is a masculine noun in the nominative case, its adjectives take the standard masculine nominative ending, which is usually -ый or -ий.