The ST BOOK is a portable computer released in 1991 by Atari. It was based on the Atari STE. The ST BOOK was vastly more portable than the previous Atari portable, the STacy, but it sacrificed several features in order to achieve this: notably the backlight, and internal floppy disc drive.
The screen is highly reflective, it is fixed to the 640x400 1-bit mono mode and no external video port was provided. For its limitations, it gained some popularity as being the most utterly portable 'real' computer of the day (slim, light, quiet, reliable, and with a long battery life, even by today's standards for all 5).
In its features it is comparable to the Apple PowerBook 100.
The ST BOOK is shipped with modified version of TOS 2.06.
Source: Wikipedia, "ST BOOK", available under the CC-BY-SA License.