So I'm making a Halloween adventure for my players (5xLvl 4), which involves them exploring an abandoned crypt. In order to progress, they need to help 3 ghosts move on. Each ghost is confined to a single room. I have 2 of the ghosts sorted, but the one in a library is giving me trouble.

I want the players to enter the library and see the ghost. They discover that the ghost is unable to talk (because library haha). There's also a large mirror in the room. The players need to find a book that's written backwards, so they can read the solution in the mirror. 

So my problem becomes, how do I give the players a clever way of finding the right book? I don't just want it to be as simple as rolling a Per/Inv check. I'm open to any new ideas (hopefully before Halloween)!

Maybe the selection of book titles could give a clue to the answere ie

The Books titels are:-

As Being True, Buy a Group, That which is Held, Ore Principles, A belief


A belief or principle that is held by a group as being true = Tenet

Answere for the Mirror = Tenet

I’m going to assume that your party is not composed of 10-13 year olds, so for any age above that I have some ideas that I’ve ranked into difficulties

1 easy) the book that they need is the only one of a particular color or the librarian is looking at the book, or it’s the only book that’s not in the right category (“the legend of [insert heroes name here]” is in the science section)

2 intermediate) for this you just choose one of the above options and then add one of these: the book is blank till a certain condition is met (like moonlight touching it or the librarian needs to stamp it) or the book will move from shelf to shelf as soon as they try to grab it (then they have to find the pattern of where it moves to catch it)

3 hard) the librarian is stamping book in morse code (or some other type of code) then once they find said secret book it’ll give them a guideline, such as “release date”, with this clue they have to find all the following books(such as others written by that same author or that’s in the same category) and add together the release dates, then the total will be the location of the book (assuming they number the books, you could also do something like they have to tell the librarian the number and they’ll point to the location), you can make the hard difficulty longer by also adding some of the options from intermediate and easy, but that’s up to you

hope this helps!

Here's a variation on something I used once, although it's not particularly difficult.  You could have an enchanted mirror that reflects things not as they are, but as they used to be: people appear younger, their clothes newer and brighter, the books and library shelves are pristine and fresh.  The secret that the PCs need to find is in a book that's so badly damaged (burned, water-logged, etc.) that it's unreadable, except in the reflection that shows how it looked when it was new.  Note that the ghost in your scenario would appear as they were in life, when reflected in the mirror.

This could be made slightly harder by adding a riddle written by the mirror, which needs to be answered in order to activate its magical property.  It could also be slightly weirder, by allowing characters to pass through the mirror into the past version of the library (or a portion of it), and interact with objects there... and maybe even affecting the "present-day" library back in the real world when they do.  That's moving away from the original idea, though.

I think producing a selection of fake book titles and, depending how many you have, you could make one of them just written backwards and let the players make the connection about the mirror after examining them (relying on the volume of fantasy book titles to obscure the puzzle), or you could have fewer titles and have one of them relating to mirrors or reflection, or something like that. 

One thing to consider would to use the mirror as the clue;

Option 1: The party looks into the mirror and sees themselves as characters from books. this would need some suspension of disbelief and will use player knowledge, EG seeing themselves as Pinnocchio.

Option 2: They see themselves as clues to the location of the book. Not sure exactly how you would go about it, though. Perhaps have them appear as a theme (I'm going Pagemaster here), so they might see themselves as monsters and have to go to the horror section, where they might find another mirror?

Option 3: The book they need is reflected in the mirror, so they have to look around the room through the mirror to find it

Option 4: The mirror is a doorway, and by passing through you find a different library, with the librarian alive. Their unfinished business is a book which fell behind a bookcase and they casually mutter "I'll get that later". Find the book and you'll finish their business!

Option 4: The mirror shows the library as it was. The cart has some books on it, so return the books to their shelves to finish the business.

The book is on the shelf in the real library but it is missing from the reflection in the mirror until (X condition met). Use the other posts to set the condition(s) you like.

Sounds like the kind of puzzle where you can leave it up to your players to come up with an answer. Anything they try that's plausible requires them to succeed on three different rolls appropriate to their strategy, or maybe three out of five. You probably improv a lot of the rolls as you go along.

For example, maybe they will ask the ghost to point to books whose titles start with the letters of the backwards book's title, in sequence. Then maybe you improv a way to make that more challenging on the spot: the ghost can't point, but can knock down books, poltergeist-like. Each time he knocks down three books though, starting with three different letters. You have to guess which is the valid letter. Many of the combinations don't form words. Maybe you give hints by having the correct books be from a consistent genre.