While it has managed to date itself now due to the amount of added content since the game was released, this book is an absolutely excellent resource for those that want to play and strategise their characters through the PvP world.
For the walkthrough, it basically gives you full details on all the missions of the main Prophecies storyline. Everything from Old Ascalon all the way through to Hell's Precipice. It has tips on the best paths through the missions, as well as details on how what the bonuses are and where to find the start of the bonus. It even gives you information on what tricks you might encounter from the mobs.
Still, by far the best resource is the loot tables and the mob tables. The mob tables detail how much damage the mobs do, what classes they are and the skills and armor levels they have. The detail what rewards each quest gives, including skills and the like. They also detail all the required items for constructing your armor, and detail all the various armor available in the game (excluding the FoW armor and anything added post Sorrows Furnace.)
I suggest you buy this AFTER you've complete the game through at least once. Otherwise a lot of the surprise is gone. ;-)