[Sir Heinrich Godfrie IV]

I have never heard of any restrictions on the amount of spells that you can learn.

However, there are restictions based upon which magic school your mage is apart of, unless you are a general mage, but even then there shouldn't be any limit to the AMOUNT of spells that you can learn.

Now memorization is another thing and this is obviously limited to the both the type of mage, whether you are a specialist or a general, and the level of your character.

Perhaps you simply have not run across any more spell scrolls that your character CAN memorize, due to spell school limitations. That might explain why your spell book levels are not fuller.

Note: I have never played a multiclassed player before, so I'm not completely sure of all the possible limitations that may occur.
I would recomend that you read your manual top to bottom, or atleast the areas of the rules of the game which still confuse you. If you want to know more about the various mage/spell schools and their limitations, then check out pages 96 and 139 of your manual. I beleive pg 139 has a table which lists "opposition schools". Hope I helped.