I've actually bungeed a duffel bag on top of the soft top when I picked up a girl with too much luggage at the airport, I wouldn't make a habit of it though.

 

I did that to a convertible mustang I had as a rental car in Florida . Maybe because I'm not a mustang fan...but either way I think this isn't a big deal at all. I would agree that books might not be a great idea, but a jacket or something...sorry I'm doin' it

 

Lay the book on top of the jacket and it should be fine for transiting. I wouldn't recommend leaving it there for long periods of time as it could do multiple things.

Stretch your top in areas that should be smooth
Depending on weight, could cause the brackets to not be aligned
Slide around and damage the glass window

Temporary is the key.

Tim

 

See what happens to the stuff if you have an accident or otherwise stop quickly. It is not a good idea to put anything loose on the back parcel shelf in cars.

 

I have seen people put travel bags back there on long trips.

 

For the LOVE of god, please DON'T put something in that storage well and then lower the top. Yes, I know the OP has a hardtop but I'm just making a general statement.

"Someone" did this, and let's just say that the rear glass window on an Audi TT took a quick trip to heaven. Needless to say, the entire top had to be replaced to the tune of $2k+