Legal Question in Intellectual Property in California

That is ridiculous! I have never heard this excuse before. You absolutely can register your book even in digital form.

In order to be copyrighted, your creative work must be reduced to a tangible form, A purely digital book, existing in your computer's memory, cannot be copyrighted. Print it out. That satisfies the tangible requirement. Then submit the print-out for copyright.

The Register of Copyright has the discretion to make rules for what can be copyrighted, and the clerks reviewing copyright applications make determinations based on those rules. If your registration was for a book and was presented in a tangible medium of expression--something that can be observed by a human being, even though it may require the aid of a machine or device--it should be copyrightable. If you used the right form (double check that you did) and submitted a copy with your copyright application, it should have been registered unless there was another reason why it could not be. The copyright office now prefers that registrations be done online and that copies be submitted as pdf files. If you did that, then you should appeal the denial. (Usually, when applications are rejected, they will give you the reason why along with the name and phone number of the person making that determination.)