First, a collaboration agreement spells out the arrangement between you and your co-author. Issues such as who pays for expenses, how your names will be listed on the book's cover, and what happens if one of you quits the project should be covered in the collaboration agreement. Second, a collaboration agreement keeps the book alive if something goes wrong between you and your co-author. Third, the publisher will insist on a collaboration agreement, so it's prudent to have the agreement completed as early as possible. This is a sample collaboration agreement.
Nothing here is written in cement.
Please feel free to modify it to suit your particular needs and circumstances. The following shall set forth our understanding with respect to our respective rights in the Work and the royalties and other considerations to which we maybe entitled pursuant to said Agreement. It is expressly understood that we hereunder do not intend to form nor shall this agreement be construed to constitute a partnership between us. The terms of this Agreement shall be coextensive with the life of the Work. No mutual expenses for which we are all responsible, shall be incurred without mutual agreement.