If you have any creature parts you only need to keep the best of each type. I kept a list of my current top part stats handy so I could sell any lesser versions before talking to the necromancer, so I wouldn't clutter up the part list.
You can also get rid of potion and enchantment formula by talking to the alchemist and enchanter (any left in your inventory after going into the alchemy or enchantment functions will be duplicates, and can be sold).
Alchemy plants and enchantment gems and minerals are easy to acquire after getting the battle tower, so you can get rid of any you are not going to use. Keep malachite gems, since there is a limited supply in the game, and ginseng (etc) for healing potions, but if you have multiple stacks of the same item, you can safely get rid of the extra stacks.
I usually kept quest related books (even if they were just added for info or atmosphere), but sold ones that show up in randomly generated loot, etc. If you check the consumable section of your inventory, you may have a stat/skill book there you can use to fee up some space.
There is a skill to boost inventory space, if you have the points to spare. You can unlearn all your skills after getting the battle tower, and get those points back once you have access to the chest there to store things.
