Place the books flat in a box or on end with the spine down. Be very careful not to distort the book when packing because if the spine is misaligned it will take that shape and will be very difficult to straighten.
A very important consideration is where the box will be stored. The box should be in a cool and dry place away from insects and rodents which might eat any of the component parts of a book. The Library of Congress web site has information about proper care and storage of books. In most cases basement and attic storage is not recommended and the target environment is cool (65-70F) and dry (45% relative humidity).