The Item Shop, added in Alpha Nymph, is a room that can be found randomly in the dungeon.
Run by an NPC called the Shop Owner, you'll find three random items on the floor with a gold price on each.
If you have enough money, you can step on any item you want and it will be yours, deducting the amount of money labeled from your total amount of gold.
While scoreboard placement is determined by your total amount of gold, it may prove beneficial to sacrifice a portion of your gold in order to obtain a good weapon or hat. The main objective of the game is to venture down to level 26 and back, and the Item Shop allows you to buy a weapon free of the dangers found throughout the various dungeon levels. This is especially useful if you manage to discover the shop on one of the first levels, but because of the procedurally generated dungeons, it's more of a lucky thing and should be acted upon.
It is not advised to buy magic books, because the spell they hold is unknown. Books should only be purchased if you have plenty of money to spend and are in dire need of a weapon.