I'm trying to make a spellbook that would give a wizard certain spells that are not normally available to wizards. 

I want them to be able to cast these spells using wizard spell slots, but i can only figure out how to make them "At will" or make it have charges. 

Can anyone help? 

Your best best is probably going to be making the book as a "Feat", rather than as a magic item.

Can you make an item give a Feat? i want it to require attunement. 

Unfortunately no, but you could create a paired item to fill that slot. Sloppy, but works for record keeping.

It is also not possible to set a Feat to have a prerequisite of a particular item.

Actually, you can’t quite do it as a feat either.

Oh, what? Weird. That really sucks. Do they really only provide 3 feat templates? Gross.

No, any feat can be a template as long as you’ve purchased access to the feat. But feats cannot have spells attached that automatically use spell slots with a “cast” button.  It would be the same as attaching the spell to a magic item. 

I have access to all content, so I'm not certain why I can only see three feat template options.

Ah, right... Magic Initiate and the like only grants the spell one per rest.

I guess the only option then is to create a custom subclass.

Having access through content sharing does not unlock content to use as a template. You have to own it here yourself.

You could do it with either a subclass or a race/Subrace/variant

Ah... that would explain it. I suppose that makes sense. Prevents someone from just cloning content directly for personal use.

If you have access to TCoE, could you take a look at the Astromancy Archive and see how it works?