Снаружи этот книжный магазин выглядит скромно, но изнутри он очень уютный.

Both words describe perspective, but from opposite sides:

• снаружи = from the outside / outside
• изнутри = from the inside / inside

They are not built in exactly the same way in modern Russian, so you usually just learn them as a pair. Literally:

• снаружи contains с-, related to from/outside
• изнутри contains из-, meaning from out of, plus внутри (inside)

In this sentence, they contrast the shop’s exterior and interior:

• Снаружи ... = On the outside...
• но изнутри ... = but on the inside...

This is the nominative case, because книжный магазин is the subject of the sentence.

• этот = this
• книжный = book
• магазин = shop/store

All three words agree in:

• gender: masculine
• number: singular
• case: nominative

So:

• этот matches masculine singular nominative
• книжный matches masculine singular nominative
• магазин is the masculine singular noun