I agree with zot. The 120 limit is perfect.
One possibility to save spellbookpoints is to agree in advance which mage you will be playing. Then you can take a decent guess which cards you will not need during this game.
I love the warlord and I don't think he is much weaker than the other mages. (maybe a little) The ability cards are pretty bad (runesmithing is decent) but they have a huge cardpool to choose from. Also, I think people really underrate talos. Just play a book where you
want the conjurations and add talos as a bonus. Talos only costs 2 sbp to include. If you don't get the time to cast him, you play something else and if the opponent gives you some time you can cast it for 9 mana or less with the construction yard. More often than not, you will have talos 4 rounds later. If they focus on stopping talos from getting out, you can progress your game or (simply summon another outpost). I don't say its a top tier strategy, but it's far better than most people on this forum think. In my experience, my opponents almost never stop Talos from getting out. Most people ignore him, or fail trying.
The helm of command is a very decent card already.
I agree on Otto though, he is not worth the manacost. Spiked pit and building you camp is possible on a small scale. But this game is an Arena fight, not Age of Empires.
If you think about it, it's not that weird that building a castle in an arena while the opponent is charging at you is not viable. And that's a good thing, or games would take 4-6 hours.