I worked at a television station years back that was designed in such a way that the lights going up the tower were powered by the separate phases of three phase AC with the one at the top powered from all three combined. This was pretty normal but what the engineer had done was rotate them at every level so that if a phase was dropped you could count the lights and quickly see from a distance that the power wasn't right. 4 lights was good, 3 meant you dropped a phase, and so on. I thought it was a pretty clever way of keeping light on all sides of the tower while being able to tell from a distance that a phase was out.

Any books recommendations for this part of history?

It's a good alternative to the discontinued SONY DPT-S1 and Onyx BOOX. (Edit: and goodereader)

The main difference based on spec is the screen, pen sensitivity, and price.

ReMarkable:

- 10.3" 1872 x 1404 (4:3, 226 DPI)

- 2048 levels sensitivity, high friction pen

- $429 (limited time offer, preorder)

SONY DPT-S1:

(discontinued Jan 17, 2017)

- 13.3" 1600 x 1200 (4:3, 150 DPI)

- pen input

- was $799 (now > $1k on ebay)

Onyx BOOX Max:

- 13.3" 1600 x 1200 (4:3, 150 DPI)

- pen input

- $760

Onyx BOOX N96:

- 9.7" 1200x825 (16:11, 150 DPI)

- pen input

- $369

Goodereader:

- 13.3" 1600 x 1200 (4:3, 150 DPI)

- 1024 levels sensitivity pen

- $699 (preorder)