All content files delivered to Apple Books must include an industry-standard MD5 checksum, a measure that helps to guarantee that the asset files received by Apple Books are correct and complete. Once received, the MD5 checksum contained in the metadata file (one for each asset file provided) is compared against the actual file received by Apple Books. If any differences are detected, the asset file and the entire corresponding package are rejected. See the Helpful Tools section of this document for tools that can generate MD5 checksums for your content.