These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
A cave in the north-west corner of Book 2 hides a treasure that you can only attempt to retrieve once, and has really nasty fail conditions (ranging from loss of Rank to instant death). However, solve the puzzle (through luck) and you can lay claim to a +10 Defence Shield, an obscenely powerful item that will make you nigh-invulnerable in battle.
It's easier than you might think to get a +6 sword in Book 5: set up shop in Aku as a magician. You'll be tasked with resurrecting a noblewoman's child from the dead, a task that requires average-level Magic and below-average Sanctity checks. The hardest part is acquiring the incredibly-expensive ingredients for the ritual... except you can get them for free as quest rewards by travelling a short distance north to the Hall Of Heroes and completing the quest there in favour of Grendal, which only requires a diplomatic approach and a single average-level Charisma check.