IBM BookManager book

IBM BookManager is IBM's legacy online documentation system, historically used mostly on mainframes, midrange (AS/400) and OS/2, although it also saw some use in other areas including AIX. IBM no longer uses it for documenting current versions of its products, but there is an immense quantity of surviving electronic documentation from the period when IBM actively used it (from the 1990s thru 2010s), particularly documentation CD-ROMs/DVD-ROMs and images thereof.

The standard extension is BOO. No public documentation is known to be available for it. The header of BOO files commonly contains an EBCDIC copyright notice (this is true even for books for ASCII-based platforms such as OS/2). The file contents is almost certainly also in EBCDIC, but is not readable beyond the header. Maybe it is compressed with unknown compression algorithms (a relative of IBM's well-known proprietary algorithms such as TERSE?). However, the data seems too repetitive to be compressed–possibly it is just some obfuscation mechanism then?

https://github.com/kev009/boo2pdf is code to convert BOO files to PDF. However, it relies on using the IBM SoftCopy Reader (for 32-bit Linux)'s JAR files and native code libraries (.so files) to actually read the file. The author of that code previously hosted a free public web service for that conversion, but discontinued it.

Versions of IBM's SoftCopy Reader (previously known as IBM Library Reader and before that IBM BookManager READ) also existed for Windows, AIX (any other commercial Unix systems?), OS/2, DOS (meaning PC-DOS/MS-DOS, not mainframe DOS), MVS (later known as OS/390 then z/OS), VM/CMS (later known as z/VM), and OS/400 (later IBM i5/OS and then IBM i)–but apparently *not* mainframe DOS (DOS/360, DOS/VS, DOS/VSE, z/VSE).

"application/book" is sometimes used as a MIME type but was never officially registered

Notes on file format
Bytes 8+: EBCDIC copyright string. Can start with one or more EBCDIC spaces (0x40), sometimes also (0xB4) (some kind of control character?)
 * Bytes 1-2: unknown 2 bytes probably flags. second byte never zero, first byte often is (but not always). probably not a size, since 0001 is a common value.
 * Bytes 3-6: these 4 bytes appear to always be zero
 * Bytes 7: unknown, can be zero, probably more flags???

The header is 256 bytes long. The copyright string is padded to the end of the header with 0x40 (EBCDIC space). However, it seems to have a null terminator (0x00) at the end of the actual text, before the space padding.

What follows from 256 bytes onwards looks like it could be some kind of compression dictionary???? Not clear.

It looks like the file is composed of 256 byte records (so file size should be an integer multiple of 256), often with some all-zeros records appended to the end.