Does the trundle include a mattress?

Usually no. The trundle is a frame that takes a separate twin mattress, so budget for one. Check the trundle's height clearance before buying a thick mattress.

How much space does the trundle need when pulled out?

The trundle slides out along the side of the bed, so leave clear floor space roughly the width of a twin mattress beside it. Measure the room before you commit.

Is the bookcase headboard strong enough for books?

It is built for books, toys and a lamp, not heavy loads. Keep weight spread across the compartments and avoid stacking dense items in one spot.

How hard is assembly?

Expect a few hours with two people. The headboard, drawers and trundle mean more parts than a basic twin frame, so sort hardware first and follow the steps in order.

Is it good for a small room?

Yes, that is its main strength. The headboard shelves and drawers replace a nightstand and a dresser, and the trundle gives a guest bed without a second frame.