.MOBI

Introduction

MOBI is the ebook format of Mobipocket, a French company whose reader ran on Palm, Windows Mobile and other early handheld devices. Amazon bought Mobipocket in 2005 and built the first Kindle’s format on it, so older Kindle books use the same structure, often with the extension .azw and with DRM. The format has been replaced by AZW3 and newer Kindle formats, and Amazon no longer accepts MOBI files for sending to a Kindle.

Mobipocket never published a full specification; what is known comes from reverse engineering.

Structure

A MOBI file is a Palm database (PDB). It starts with a 78-byte header holding the book’s name, dates, the type BOOK and creator MOBI at offsets 60 and 64, and the number of records, followed by a list giving the offset of each record. Record 0 holds a 16-byte PalmDOC header (the compression method, the text length and the number of text records), then the MOBI header starting with the bytes MOBI, and usually an EXTH block of metadata such as author and publisher. The book’s HTML-like text follows in records of up to 4,096 bytes, usually compressed with PalmDOC’s simple LZ77 scheme, then images and index records.

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