This well-made tome is always of small size. 

One will typically be no more than 12 inches tall, 6 inches wide, and 1 inch thick, and some are a mere 6 inches in height. 

All such books are very durable, waterproof, iron and silver-bound and locked, and gain a +3 bonus on their saving throws (as leather or book).

 The pages of such a book will accept magic spells scribed upon them, and any book can contain up to 45 levels of spells. 

The book is thus highly prized by magic-users of all sorts as a traveling spell book. 

It is totally unlikely that such a libram will ever be discovered (randomly) with spells written therein, because inscribed or partially inscribed works of this nature are kept carefully by their owners.