How to get Spells

[Hassadar]

Het there,

I know this seems to be a stupid question, but im clvl 30 now and I'm missing some spells like frozen orb. So am I just too low lvl, or do I miss something on how to get em all is like in Diablo?
Can someone pls explain to me how I get spell spooks most effectively?

Thanks in advance.
Oh, if I missed some obvious info about this I'm sorry.

[Ugily]

Buy spell books from the Witch if any are available to learn, but don't waste gold buying spell books that you can't immediately learn.

Get past the secret passage quest and there will be numerous higher level spell books.

Defeat Diablo and begin game play at Nightmare and Hell difficulties. Higher level spell books will drop.

[Hassadar]

Hmm...it seems im doing something wrong here. I'm in caves on nightmare now and still missing some spells, is that supposed to be correct?
Thanks for the answer!

EDIT: a friend of mine is some levels behind me, hasn't even defeated Diablo yet and has more spells than me (he's warrior I'm mage)

[Ugily]

Just keep making Cathedral and Catacomb runs and checking with the Witch. Stockpile spellbooks in your stash to reach level 15 each. Once you get a bunch of +Mag equipment, Mag potion, Mag shrine, and Random +Mag shrine all at once you can learn a bunch of spells at a higher level than you normally would be able to obtain.