What are the optimal ways to practice? Daily. Sketch something daily. I'd suggest a variety of subjects from photos. It's OK if you don't finish a sketch every day, so long as you put in at least 15-30 minutes most days. Most adults that practice daily see huge improvements in the first year or two - they surprise themselves with their progress. Some of the sketches will just be practice drawing shapes and shading.
What are the best books and reference materials to go off of? This is truly an individual thing. There are many youtubes out there. Alfonso Dunn makes some decent drawing videos. There are many books out there: whichever seems to speak to you is the one to get. If you can, get one in which you are interested in the subject. Different subjects will have different bits: Portraits and human figures are different than landscapes, which are different than still life. I'd honestly suggest a combination of references and subjects at first.
How do I learn to self-critique? This is a really difficult thing to do, some of which will simply come to you as you learn more about art. There are some tricks, such as taking a picture and viewing it in black and white to see contrast. Studying color theory helps as well. Study other people's art: Read art advice from artists you personally think do good work. Some folks look at their pictures upside down: I personally will make sure to look at the picture from afar. Many set their work aside for a few days to look at it with "fresh eyes". I'd fully suggest things like a facebook group or Wetcanvas forums for an outside view.
What other questions should I be asking? How to protect yourself from repetitive motion injuries. Tennis elbow, carpal tunnel, shoulder issues, and other such injuries plague artists. You'll want to make a habit of taking breaks during longer sessions, stretching your hands and less frequently, your body. You'll want to have good posture for the most part.
The last bit of advice is to learn to be kind to yourself. Most folks tend to see all of their "mistakes" when others won't. It'll take time to both train your fine motor skills and things. Good luck and have fun on this journey :)