Doctor Who: The Television Companion by David J. Howe and Stephen James Walker was my first thought here. I must have read it all a dozen times over, bit by bit. You start by looking up a transmission date, then begin reading the analysis, and next thing you know it's time to go to bed and you're wondering where the day has gone.
Other Doctor Who books were contenders too - Planet of the Daleks, The Discontinuity Guide, The Claws of Axos (which I got in exchange for some marbles at middle school), Day of the Daleks (77p at Millers), and Image of the Fendahl (borrowed from the town library three or four times and read in a single night each time).
Then I thought about novels, like Planets for Sale by A.E. van Vogt and E. Mayne Hull (which led me to read loads of tedious A.E. van Vogt books before realising I should probably have been looking for E. Mayne Hull books), The Duelling Machine by Ben Bova, The Time Trip by Rob Swigart, The Stainless Steel Rat for President by Harry Harrison, or Biggles in France by Captain W.E. Johns.
But no, it had to be the second edition of The Slings & Arrows Comic Guide, edited by Frank Plowright, a wonderful series of critical essays surveying American comics from the 1930s to the present day. The other books I've read several times, but this book I've never stopped reading. It was published in 2003, over 3800 days ago, and I'd estimate I've read from this book on at least three quarters of those days.