It's great to hear you took the critique you received well! The above books are good reads, but I think you'd save money and learn faster simply by googling exposure triangle, rule of thirds, photographic composition, etc... read until you feel you are comfortable with terminology and the basics of how the basics works such as aperture, shutter speed, ISO, f/stop (memorize them, it's actually pretty easy), focal range and how it affects compression and perspective, light quality, depth of field, noise, and any other common terms you may run across but aren't sure about.
It sounds like a lot to learn, but it really isn't because everything tends to tie together. I bet after four hours of reading various articles on the above topics on the internet, you would be well on your way to creating much much better images. After a few months, you may even look back at the images you like now and ask yourself "What was I thinking?!?" At this point you'll realize how far you've come. I have images like this, I bet we all do. We hang onto them because we once thought they were great, but now we realize how bad they are. It's a great confidence booster.
Once you feel you've done enough reading (perhaps you already have?), grab your camera, and carefully compose a well lit portrait of something/someone you can control. It can be a person that is willing to be your model, or a stuffed animal. Keep it simple and just do a head & shoulders standard portrait. Think about light quality, light ratio between subject and background, and composition. Are you handholidng? Shutter speed must be sufficient for your focal length. Tripod? If nothing in the frame is moving then for the most part shutter speed is irrelevant in relation to sharpness. How much depth of field do you want? Use your DoF preview button if you have one. Think about these aspects before you release the shutter.
Take the shot. Does it look how you envisioned it? Are you happy with the result? If not, really think about why not. Is there anything you do like about it? Remember to think of things like light on the subject, light on the background, light direction, composition, background color/objects/texture, focus, exposure, white balance. Which of these aspects of the image do you like, which do you not? What can you change to improve a dislike?
Then, when you feel like you're happy with the shot (doesn't have to be perfect or stellar by any means), post it here, let us know you're thought process, what you like, what you think needs work, and we'll let you know how we feel about it, and you will almost certainly have something brought to your attention that you didn't think of... perhaps a problem with the photo, or an easy way to make it better. You need to be your worst critic though before you post for C&C or it's going to hurt more than it should. Don't expect any particular outcome, but offer it for C&C with intent to do it again while taking into account the suggestions that we make (or perhaps we will think it's solid and no changes need to be made, at which point you'll be ready for another lesson, another setup, another type of photo, etc.
