The Summon Skeleton spell is pretty underwhelming compared to Skeletal Wall. Rank 4 Summon Skeleton is available at level 15 and lets you summon 4 level 16 corpses. If you wait one more level, you get Skeletal Wall which summons 5 better skeletons. There's no reason at all to take Summon Skeleton Level 5.
The final boss is seriously way too easy, especially compared to Josephina. Since defeating Josephina is required to beat the game already, you can afford to buff up the big bad some more (you can probably also tone down Josephina some, too).
Only the Wizard's Spellbooks are available on bookshelves. It would be nice if you could also get Warrior Tablets and Survivor Scrolls from bookshelves as well, although I can understand for realism reasons why only the books are available.
Not all the Wizard's skills are available in Spellbook form, too. Many spellbooks have the wrong name. The "Elemental Bolt" book is called "Sparks" and actually increases "Elemental Hail". The "Magical Might" book is called "Heaven's Blessing".
Spellbooks ignore level requirements (not that I'm complaining, it's nice to have Rank 5 Elemental Hail [Required level: 48] at only level 11).
While not a problem, it would be nice if your Mana regenerated slowly. Perhaps it could be balanced if Mana only regenerated when you have full stamina (and it doesn't regenerate when the game is paused, obviously). That might be too unbalancing, though.
Some tooltips or loading bar tips might be helpful here: With the 'Know Creature" skill, you can see the target's Offense, Defence, and Chance to Hit. That's fine, but I don't understand what the Offense numbers mean in terms for the enemy's chance to hit me and I don't know the difference between the Enemy's Defense and my Chance to Hit. Providing those numbers doesn't mean anything to me without explaining what them mean in layman's terms.
The final boss is seriously way too easy, especially compared to Josephina. Since defeating Josephina is required to beat the game already, you can afford to buff up the big bad some more.
Bug: The bookshelves in the basement of the Merchant's guild are linked together. There are several bookshelves and chests which all point to the same container. If you mouse-over each linked bookshelf, you still get a chance to spawn a new item, though.
Bug: The Poison Cloud spell can be cast near friendly NPC's to kill them without turning them hostile.
Bug: You will fix the broken portal stone to escape the Secret Developer Area, right?