The answer depends, in the first instance, on whether your father assigned his rights or granted an exclusive license to the company that published the books. If you cannot find any contracts, you might want to contact the publishers and ask them for copies. If no rights were transferred, or if the rights reverted back to your father before he passed away, the manner in which the rights descended to the heirs can be complicated. Even if he transferred his copyrights to publishers, the Copyright Act allows authors or their heirs to terminate this transfers after a certain amount of time. Also, if the books are out of print the publishers may be willing to voluntarily transfer the rights back to the author's heirs. You should consult with a qualified attorney, preferably one with copyright experience.
Views expressed are mine and do not represent the views of Arent Fox LLP (or its partners, employees or clients) or the New York State Bar Association. This answer is provided for the purpose of furthering the analysis, understanding and discussion of copyright law. This answer is not an advertisement for my services, nor is it given for the purpose of soliciting my retention as your lawyer. Nothing in this answer is intended to serve as legal advice, nor should you rely upon this answer to serve as legal advice. If you need legal advice, please seek out and retain qualified legal counsel to provide such advice based upon your specific facts and circumstances. Nothing in this answer creates an attorney-client relationship with you. To be my client, you must be a client of my firm, which means that we must make certain disclosures to each other, check certain facts, make financial arrangements, and enter into a written agreement with and approved by my firm.