In terms of intellectual property "public domain" refers to items that have no restrictions of use due to expiration of legal controls (patents, trademarks, copyright)
Pubic domain occurs naturally when a copyright expires. In theory, a copyright owner can intentionally disclaim any power to enforce a copyright, making it as if it were "public domain".
Read it, in private. Quote limited excerpts in reviews or educational materials. Re-sell or loan it (or, notably, burn it). Virtually all other uses would require permission from the copyright holder.