Offhand items come in 3 types and have generally have 3 different styles.
Shields (and variants); Increased armor and magic resistance, faster healing, improve effect from being healed.
Torches (and variants); More Hit points and improved hit point regeneration, haste and speed buffs, reduced cool downs.
Book (and variants); More energy and improved energy regeneration, faster spell casting, increase cast speeds.
So choose one that complements your build, if you want to absorb damage then shield, if you want to heal or deal out spell damage then books, or somewhere in between then torch. Generally the items are intended to pair with their weapon and armour sets.