Learn Italian: L'insegnante non è qui e il libro è difficile. - The teacher is not here and the book is difficult.

The conjunction e (and) usually stays the same. However, if the very next word starts with a vowel—especially the letter 'e'—it often changes to ed to make it sound smoother to say (for example, ed ecco). Since the next word here is il, sticking with just e is perfectly fine and the most common choice.

Is L'insegnante a male or female teacher?

We actually can't tell from this sentence alone! The noun insegnante ends in -e and is the exact same word for both men and women. Because it starts with a vowel, the definite article "the" shrinks to L' for both genders. You would need more context to know if the teacher is male or female.

Since libro is a masculine word ending in -o, why doesn't the adjective end in -o too (like "difficilo")?

Not all adjectives change their endings to -o or -a. The word difficile belongs to a different group of adjectives that naturally end in -e in their dictionary form. These adjectives use the exact same -e ending for both masculine and feminine singular nouns.

Does non always go right before the verb, like in non è?