Books reward preparation. Focuses reward a rhythm. Magic training gives both something worth preparing for.
Bring a book
Expanded Reserves gives an equipped Book more maximum mana as your training improves. The extra capacity starts empty, so swapping the Book on and off will not solve your resource problems. Studied Casting rewards a quiet three seconds with preparation for a cheaper spell.
Recollection recovers mana over a stationary three-second channel. Moving, taking damage or starting another action can interrupt it. You keep what you recovered, and the cooldown stays spent. Find somewhere sensible to read.
Find a rhythm
Focus training speeds eligible casts while keeping their recovery intact. Flow State rewards finishing a spell and following it with a different one. Books have Memorised Sequence, which can reduce the next different spell's cooldown, with exclusions for things such as ultimates and Resurrection.
These benefits care about the equipment actually in your offhand. Drawing or stowing its visual does not change ownership. Swapping gear can clear preparation, and a spell already admitted keeps the timing it was given.
Make the next cast matter
Healing and Offensive Magic now apply their trained output benefits through the spells that own them. Overchannel prepares an empowered offensive cast with a mana surcharge. Healing empowerment and the buffing paths have their own preparations and upgrades.
Enduring and Renewed Enchantment track the original caster and effect duration. Renewing a buff extends the appropriate existing effect without replaying its instant rewards. You still have eight ability slots, so all this preparation needs a little planning before the fight starts.