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The OP never stated he was going to use the hammer as the Boded Item. Where are you getting this hammer from? I got confused reading this....
Well, he said he'd "probably" use a ring or amulet, but mentioned the warhammer, so we were discussing the possibilities if he used said warhammer as the bonded object. He said to have discussion so he could see ideas bounced around, so that's what we were doing. :P
My opinion has solidified in the weeks since this was posted, especially for multiclass characters like the OP is describing (which would have spellbooks that would tend to be more complete than straight-classed wizards of the same wizard level). Being able to cast any spell of any level from your book is just too versatile of an ability to turn down.
I second the above. Feather Fall when it's really needed. Water Breathing or Walking. Grease, perhaps on yourself if something looks like it is a threat to grapple you. Some sort of "help get the party away" spell. At least early to mid levels it would be hard to have a wand or half wand for every possible situation.
Then there's always just plain one more of your "most powerful" whatever spell. Or a powerful spell of a different element damage than you normally use based on what you are fighting. Bonded item amulet will be my choice whenever I play a wizard. And I plan on playing a straight-classed wizard at some point. You get so many spells in that book. Any one, any time is very stout.
Yes, and you get the enchant item feat with respect to that item for free once you meet the level requirements.
For that reason, I'm probably going to take a ring the next time I make a wizard because I never take Forge Ring and often have a ring slot open in the middle levels. It means you have to wait an extra 4 levels to use that particular part of the bonus but when you get it, you can make it Spell Storing or Blinking or Invisibility or whatever.
If you go with a weapon for your bond, you don't get proficiency with it, do you? Got an idea in the last day or so of an axe-wielding Ulfen wizard.
EDIT: Actually, nerfed though it was, I could get away with Heirloom Weapon if the DM let me use the weapon from that as the arcane bond (and subsequently upgrade it to a masterwork weapon, not that it'd see much use in the hands of a wizard).