Are clothing designs described in literature protected by copyright?

If many of the specific details of the design of the clothing are taken from the work of fiction, then it might well be a derivative work, If it is, the copyright holder's permission would be required to make or sell it (or otherwise distribute it). The more vague and general the description, and the more original features not in the source, the less likely the clothing is to be considered a derivative work. But if key features are clearly taken from the description, then it would probably be considered derivative.

A derivative work normally gets its own copyright on its original aspects (providing the source is used lawfully), but is also under the copyright on the original, and the permission of the copyright holder on the original is needed. Making a derivative work without such permission is infringement (unless possibly fair use(in the US) or some other exception applies). An infringing derivative work is denied copyright protection under 17 USC 103.

Whether a particular work is sufficiently derivative to be under the copyright of the original depends on the specific facts, and is beyond the scope of this forum.

It's questionable, because if you design your own visual interpretation of the T-Shirt then it isn't necessarily the one from the book and thus your art has it's own copyright. However, if the current Copyright Holder and Possible Trademark owner is selling the shirt it could be an issue because yours is not official but being sold as one. If you're making it for non-sale and just cosplay, than you have a better arguement.