I have not really found good advice on this issue (human advice not resources). So let me try to articulate my question in hope to create a discussion:
I turned to linux at 2006 after using it first at the university (openSUSE and some scientific debian based distros). I then went for ubuntu, my first installation was Hardy (8.04) which was awsome! Ever since (2008) I have stayed with ubuntu and installed it to about 20 other machines, for friends, family and for me. In my humble opinion Ubuntu has excelled in becoming super user friendly in very few years and has now a beautiful and modern GUI (the older Gnome was a classic too).
However, although I consider my self an experienced user and I always have some terminal open because I love tweaking stuff, getting feedback and troubleshooting, I feel that I hit a plateau in my lurning curve of linux. I now have familiarity with a variety of commands and manipulations, from constantly trying to push the functionality of my Ubuntu, however I am totally dependent on blogs, forums and documentation for even relatively simple stuff. I understand enough to change the commands proposed and figure out where things go bad. I generally get things to work, but I almost never manage to simply open the terminal and do something other than browsing and file manipulations, i simply do not remember the commands since they are structuressly scattered in my mind. I have lately noticed that I install programs that are simply a GUI for a command and I always find that I understand better what is happening when I am at the terminal (you see all the errors for example so you can guess wher things go wrong instantly instead of searching for error logs or where that or this option is in the graphical menus).
In other words, I don't really feel I know any linux, I just learned how to be a very good user of Ubuntu. I want to take it to the next level but I am not sure what that level is. I am not an IT professional (i do a phd in mathematical physics) or want to be one, so courses on how to become a linux administrator is maybe not what I need. I KNOW that this is a very vague question, I am not really looking for do this and that advice and that is why I do not mention any specific software, I would simply like to hear from other people what they did when they arrived at this point, to round up the knowledge gathered from years of terminal usage to troubleshoot and personalise Ubuntu to their needs.