I understand it can be difficult to take ove a class from another teacher. You can feel like an outsider coming in. Your task now is to create a new and more positive atmosphere that will enable students to work together.
You need to start by building up respect and trust. The students need to respect the opinions of others in the class and feel that they can trust others not to insult them or mock when they speak. Start by leting them know that nobody is allowed to mock or laugh at another person's efforts and enforce this rule whenever they break it. Whenever you can, take the opportunity to talk to the class about the need for respect. Teenagers usually understand this and agree with it; it's just a case of altering their behaviour by constantly reminding them. With this trust and respect, they'll have the basis for working together.
Next (or at the same time), you need to do more activities that bring them together. Focus on cooperation, not competition. You can do info gap activities that require students speaking to others to complete a task. They can also write and carry out questionnaires in class. You can also do running dictations as cooperation is required to complete the task. Consider project work which has students working together in groups. Also change the groups frequently; otherwise you'll have a situation where sub-groups form that are cohesive but the whole class is not united.
Also try activities that get them huddled up together. physical closeness can bring them together as a group. An example might be to write out instructions for homework on a piece of paper and pin it to the notice board. Students need to stand around it to copy it down. The same can be done for classroom activities or for questions for listening and reading comprehension. You'll probably find that as they group together and some can see and others can't, they'll start helping each other.
Keep going with these cooperative activities and don't have competitions or prizes for the best, fastest etc; even if it does seem like fun. Competition can be used when they're better united as a group.
As for the coursebook, you probably need to continue using it. I guess it's part way through the year. Adapt exercises whenever you can and make them more challenging or more adult-like. Also consider using project work to supplement the course book.