Explore.

There must be some kind of loot or reward for beating the boss, and you did just technically beat the boss, so surely you can reap the spoils with a clean conscience... Well, ignoring the fact that beating the boss is probably the worst thing you can do as a dungeon monster...

You walk to the altar, studying it and the bookshelves for anything of interest. You flip through some of the books, and find that they are all written in a language you don't recognize. You guess it's either some magical language or utter gibberish. You put the books back, disappointed that they weren't skill books or something, and look around the area some more. The altar doesn't seem to have any use to you, and you see no treasure chests either. But then, looking back to the bookshelves, one book catches your eye. All other books on the shelves have covers in tones of brown and dark red, but this one book is the only one that is instead bright red, standing out significantly.

...It's on the top shelf though. Still perfectly within reach for a human, but not so much for you. Muttering to yourself, you drag yourself up on the altar which is only barely short enough for you to successfully climb, and from there you reach for the shelf on the side of the altar, where the book is. Your fingers brush against it, but you can't pull it out from this angle. You lean forward more, your other hand trying to grip the stone bricks of the wall for some minor form of support, and then finally you curl your fingertips around the back of the book. You slowly and steadily slide it out, until you predictably lose balance and fall off the altar. You pull the book with you, along with most other books on the shelf which rain down on you painfully. You hope they don't deal any actual damage to you.

You stand back up and grumble to yourself while looking around for the book you wanted. You're confused to see that it's not among the books on the floor though. You look back up on the shelf and see that it's still there, only pulled out halfway.

Just as you're about to groan that you'll have to repeat your attempt, you hear the sound of stone grinding against stone. Turning towards the sound, you see a hidden exit has opened. So that was it... The book is a secret lever, made to stand out so that dumb heroes can find it easily.

You hesitantly walk towards the opening. All you can see within is a long, dark tunnel. Part of you is curious to see what it leads to, and another part is shuddering at the idea.