Very informative and super helpful to have all of the specifications and sample files in one document. This is really taking shape.
I have a few suggestions:
---On the hardware side you are getting really indepth and this is helpful to those who may already have some arduino experience but are now interested to try uiflow. However the code examples you are using for the units are more for beginners and I think starting off the book with some very basic code concepts like setup, loops, delay, etc would really help support those samples. At the same time you also might want to have some examples of more complex code to show those experienced users just how capable uiflow can be.
--- I really like that you describe i2c, digital, analog, and uart units. I think it would be great to have an visual index of all units and divide it up, like the periodic table, according to those categories and then have a chapter on each of the 4 types.
-- Start off the unit chapter by showing how a unit is selected and loaded into uiflow and how it adds new blocks according to the selection. This goes over the general workflow of using a unit. I think this can help prevent you from having to explain how to load each specifc unit and focus in on what the unit does.