Spellbook of Spellstoring

The problem with mages and wizards is that they have to carry around spellbooks full of their spells they have researched so they can study them everyday.  Kinda like they are studying for a quiz everyday, only with real world applications.  If a mage lives long enough they acquire quite a lot of research and spells, thus translating into many books that are likely to get lost.  Master Marba was a man who liked to travel out of necessity.  Something to do with his relationships with women, the history books think, and thus needed his stuff to be easy to transport.  Thus, after much research, he came up with a single book which could hold all his spells.  The book appears to have fifty or so stout pages which are fire and water proof.  The only pages that has writing upon it, though, is the first few pages.  These pages are a list of spells, but no spells follow.  If the mage knows the activation phrase, which is created with the book, they may speak the phrase and the spell name and the spell will appear on the next few pages until the book is closed.  To write a spell in, first the phrase has to be spoken, then the title of the spell written in the list and then the spell copied into the next few pages.  The phrase is spoken again and then the book is closed.  There after it will be in the book.  No one knows for sure how many spells can be held in a spellbook of storing, but they are certainly a prize for any mage.