The recommended spell books aren't that badly balanced. Make sure you use the most recent recommended books for the core set mages (available in the
latest version of the rulebook).
As for the Warlord feeling under-powered, well, that won't change much until you get the Forged in Fire expansion. Or play Domination. Bloodwave Warlord is
awesome in Domination - with cards from Domination, of course - e.g. Altar of the Iron Guard for free guards, Slaknir for Elusiveness on all your Goblins, Galaxxus is cheapest for Warlord, ditto for Gravikor when necessary, then use Construction Yard with your Barracks and Garrison Post, and maybe Archer's Watchtower to fire arrows over walls... and so many choices of earth and war school Walls!
But I digress. For starter books for Arena mode only, the recommended ones aren't that bad.