Yes, If you choose to swap out a piece of equipment, you can and it can go back into your book. if you have a hand weapon and cast another hand weapon, you could have both out or put one back, as long as one hand is free.
Most people will carry 2 chest pieces in a book because if one gets corrode markers on it, you can swap the armor, and the corrode markers fall off.
Yes, battle forge costs 8 mana, so theoretically you need 8 rounds to "pay for it" but it generates its own mana, and allows you to equip a weapon or armor with out using one on your quick actions. That can be very useful because you only have 2 actions a turn, and spending one of those to put on leather gloves, instead of force pushing an enemy into / Through a wall is cumbersome and not efficient use of your actions.
