Psion began in 1980 and delivered its is first hardware organizer in 1984. The Organizer II, a vastly improved version quickly followed in 1986. It began with a 2 line display but later versions increased this to 4 lines of 20 characters. Certainly not a reasonable device for eBook reading but in 1993 the Psion Series units were produced. These were clamshell palmtop devices with a keyboard on one side and a screen on the other.
EPOC is a single-user preemptive multitasking operating system, written in Intel 8086 assembler language and C and designed to be delivered in ROM. It was the most powerful operating system on any organizer of its day. It would later be replaced with the Symbian OS for Psion's new line of 32 bit PDA's. It was designed specifically for the PSion Series 3 devices, but continues to this day as the OS in the Workabout MX, which is a rugged portable data collection terminal.
 Series 3
The Psion Series 3 was produced from 1993-1998. The 3A with its improved 480x160 three level gray scale display provided the first reasonable size display for the Psion (the original was 240x80 black/white). It ran a new multi-tasking operating system called EPOC. The processor was 8086 family based which made it similar hardwarewise to the HP 200LX but the operating system was more powerful. EBooks were supported on this device using the internally developed TCR format. This was a compressed text database format similar to the later released PalmDOC format.
The Acorn Pocket Book II was a rebadged Psion Series 3A unit with 256K of memory (expandable to 512K), plus plug in SSDs (Solid State Disc - battery-backed RAM, FlashROM, or mask-programmed ROMs) that act as 'discs' or read-only storage for commercial applications. It had 2 Meg of ROM built in with additional applications as compared to the standard 3A's 1 Meg.
 Series 5
The Psion Series 5 devices were released in 1997 and included a completely new OS. Initially it was called EPOC 32 bit but was later changed to Symbian OS.
The CPU in a Series 5 is a 32-bit RISC-based ARM710T processor running at 18MHz (Series 5) or 36MHz (5mx), with 4, 8 or 16MB of RAM. It's powered by two AA batteries, typically giving 10 - 20 hours of use. The display is a touch-sensitive, backlit half-VGA (640x240-pixel) LCD with 16 greyscales. The keyboard is generally considered to be among the best for its size, with large-travel keys and touch-type capability. Both RS-232 and IrDA (infra-red) serial connections are provided, along with a speaker and microphone, giving dictation ability as well as playing music. External storage is on CompactFlash.
 Beyond Series 5
This is really Symbian OS.
 Other Products
The also made the original Psion netbook from which the current products get their name.