I see a mistake in those circuit cards from Adafruit. On card 26, that is technically a current -sink-, not a current source. And the current will rise with temperature. A current mirror would be more stable with temperature, and could go as low as about 0.6V on the output.
If you want different projects/circuits to play with your Arduino with, try the Arduino Cookbook by Michael Margolis and published by O'Reilly - just google "Arduino Cookbook",
To add to the Very fine collection so far, placing Heavy Emphasis on the Pighixxx collection, I'd like to nominate the US Navy Electronics training books.
They are available in many used book stores and some libraries in the US and were my main reading in my early to mid teens as was the Radio Amateurs Handbook published by the ARRL. My first one didn't cover much on semiconductors as it was written in 1951 and solid state wasn't used at all back then..
it is useful to have several editions as the circuits do change year to year. I still use a circuit I found in a 70's edition that wraps a PNP power transistor around a linear regulator.... With a small negative power source (I used a 40106 hex inverter and a 78L05 as an oscillator and driver for a 1N5818 Schottky diode to provide a small negative bias and an LM317M for a 0 to 15V bench power supply that still works well after 18 or 19 years of nearly full time use...
The RSGB handbook is a fine source of theory as well. The attached picture is from a 2010 copy I found somewhere.