Legally, a copyright is considered a piece of personal property and transferring it from person to person is subject to both state and local property laws in addition to federal statutes. Except in cases of inheritance, transfer of exclusive rights must be made in writing and signed by the copyright owner. Non-exclusive rights can be transferred without a written agreement. In other words, you can allow somebody to print your work, but they won't have the right to sell it without your written permission. A legal contract is the usual method for copyright transferal. It's not necessary to file a copy of this contract with the U-S Copyright office, but there are certain legal advantages to filing, particularly in settling disputes over the rights to a given work. You should consult an attorney familiar with copyright law to assist you in preparing a legally enforceable transfer.