How can I maintain a common task list and use it for all the projects?
A common task list cannot be created in Zoho Books.
As a workaround,
Create a project with the common task lists you wish to have.
Under Tasks, click on Import tasks from existing projects whenever you create a new project and wish to have the same set of tasks. Select a project from the drop-down in the pop-up that follows and select Import.
Now the new project will contain all the tasks from the existing project.