Which mockup images have transparent backgrounds?

The downloaded pack includes both for each shot:

• Transparent PNGs — no background, so you can drop the book onto any color, photo, or web layout.
• Neutral-background versions — the book on a clean, simple backdrop, ready to use as-is.

Use transparent when you're compositing into your own design, and the neutral version when you just need a finished image. Both come in every download, so you're covered either way.