Big mistake.
If you are new and in a position of authority over other instructors, I would say it's tremendously unwise to make a decision like this on your own.
I have to disagree with our lovable rodent on this one. No, when you are in a position of authority, you get to make decisions on your own. Or you can turn the decisions over to others. Or you can form a committee. You decide how much consultation you want to do with others before you make a decision. You are in charge. Even if you turn a decision over to someone else, or a committee, you are in charge because you gave them permission to make the decision. But, if you do turn a decision over to someone else, you have to be willing to live with their decision, because stepping in and reversing their decision creates many more problems than if you kept the decision making for yourself in the first place.
What ever direction you take on a textbook, its all good, as long as you stay in charge. You get to decide.
You would have garnered HUGE points had you asked the adjuncts to form a committee and simply oversaw their decision making. (I did say oversaw, not override). Committee work, though dull as dishwater, will keep you from frying your chances to command real authority where it matters.
OK. Would the adjuncts be paid for this committee work? All of them? Is committee work in the adjunct budget? We are not paid for prep time, which is a little more crucial. Are you going to increase the adjunct pay, to justify having them do committee work? Or would the adjuncts be willing to donate their time to study a pile of text books? (I was on a media selection committee and it took many hours a week over a couple of months.) Do they just want to give you an opinion without putting any time in on it?
Maybe the textbook decision is one of those real, command decisions that matters.
Now you've got an us v. them situation that will NEVER (ever) resolve while you are on t/t track there.
Sorry to be so ugly about it, but this would be the truth at any cc or u I've ever worked. Not good. Not good.
If the adjuncts are foolish enough to perpetuate an us vs. them dynamic, you have to win and put the whole thing to rest quickly. Get some new adjuncts if you have to. I'm an adjunct. I would not fight this battle. I would pop in to your office and ask you some procedural question or advice on the course, just to send a message that I'm on your team. I think most adjuncts will just get to work preparing for the new text.
In my day job, the new department head decided to implement a new computer system. There were lots of loud, angry protests, but the manager just calmly said, "You will start using this and get trained on it by such and such a date." That ended the discussion.
There are many private sector industry leaders who would tell you that even if you loved the old text, it would be a smart move to step into your new job and change the textbook just to establish your authority. That is why new industry leaders rearrange work teams or move people's offices as their first act of command. They are sending a message that a new person is in charge, and the new person will be making new decisions. It is done all the time--deliberately as a power message. A good leader has to decide what is good about the old system that needs preserving, so he/she is not reinventing the wheel or throwing out years of collective knowledge with each leadership change, but in private industry a new leader always chooses something highly visible to change. Sometimes they wait six months to get the lay of the land, sometimes they do it right away. It may be a major reorganization or it may just be new vending machines in the lunch room, or potted plants in the foyer, but they do something that shows.
They also don't care if they have to replace a few employees who don't accept the new leadership and want to fight it. Better to replace them now than after a battle. The leader stays in charge.
Changing a text is a good strategy because it is a substantive change, it is visible, but it doesn't reorganize the whole program when you don't want to or are not ready to do that. No one's job changes.
My ex was a private industry manager. (He was an excellent manager, just a lousy husband.) I learned a lot from him. I also learned a lot working in and observing a toxic department that fell apart. I escaped with more than a few psychic scars and a lot of lessons.
You are on the right track. Be understanding but don't be apologetic--that is what was good about larryc's suggested email.