For spellcasters. This appears as a thick leather-bound book, with copper points at the corners and gold leafing. It will radiate slightly of magic, if tested for, but will not appear to be Cursed in any way. This book is filled with thousands upon thousands of details, articles, and other pieces of reference material, all in the native language of its possessor. Should it be passed from one character to another, it will now appear to be written in the native language of that character.
While testing it out, there is a basic 50% chance that this book will have an entry on whatever the character decides to look up, no matter how obscure. If the entry exists, it might be a short, one or two line explanation, or a long article, including facts, figures, and even illustrations. Minor, or simple entries, or ones that aren't very important to the adventure at hand (GM's decision), might be perfectly fine, but invariably, whenever the character looks up something that pertains to a matter that has true importance, especially for whatever grand adventure the character is currently involved in, there will be at least one or two pieces of information about it in the book. These details will always be presented as though they are absolute fact, but, in reality they will be incomplete, misleading or even outright false.
This book will present true facts whenever they don't really matter to the adventure, or to anyone's survival, and then outright lies whenever these things do. Mixing truth with falsehoods makes it difficult to know how unreliable the book is, and characters may consult it for a long time, following the wrong lead over and over, before coming to understand its true nature. The book may be discarded at any time, though perhaps not before it has caused a tremendous amount of hardship.
Optional:This could easily be a book targeting clerics, with it taking the form of a lost volume of holy lore.
Leather Book Missale Romanum Decreto Sacrosancti Concil II Tridentini Editiio XV Juxta Typicam Vaticanam 02 by PattySixty under a Creative Commons Attribution-Share Alike 3.0 Unported License modified in Inkscape by David