While holding this 50-page tome*, you can use your reaction to absorb a spell (not a cantrip) that is targeting only you and not with an area of effect. The absorbed spell's effect is canceled, and the spell's energy -- not the spell itself -- is stored in the blank white pages of the book as arcane runes (silver if the spell is cast by an arcane spellcaster, gold if by a divine spellcaster). The energy has the same level as the spell when it was cast, and takes up a number of pages as levels it has. The book can absorb and store up to 50 pages of energy over the course of its existence. Once the book absorbs 50 levels of energy, it can't ever absorb more. If you are targeted by a spell that the book can't store, the book cannot have an effect on that spell.

When you become attuned to the book, you know how many levels of energy it has absorbed over the course of its existence, and how many pages of spell energy it currently has stored.

If you are a spellcaster holding the book, you can convert energy stored in it into spell slots to cast spells you have prepared or know. You can create spell slots only of a level equal to or lower than your own spell slots, up to a maximum of 5th level (using 5 pages). You use the stored levels in place of your slots, but otherwise cast the spell as normal. For example, you can use 3 level pages stored in the rod as a 3rd-level spell slot.  Once the energy has been used, a page becomes blurred gray (with silver runes) or yellow (with gold runes). 

A newly found book has 1d10 levels of spell energy stored in it already. A rod that can no longer absorb spell energy and has no energy remaining becomes nonmagical.

*It's a very dull-looking book, except for the image resembling a rod of absorption on the front cover.  

Notes: Utility, Warding