Just a random thought: Solving chess positions and problems only in your mind without a board is a useful method of chess training. It increases your calculation ability and helps you avoid missing long moves on the ranks, files and diagonals. One thing beginners, and even stronger players, fail to appreciate is the geometry (lines, diagonals, etc.) of the chessboard itself. Very few books even touch on this subject but masters do not actually “see” future positions in their mind’s eye. The see patterns, lines of force and “auras” of the pieces.
I discovered a site called Chesslodge that features some free lessons. The authors, IM Miodrag Perunovic and GM Alex Finkel, cover different topics on strategy. They also offer private lessons in pdf format and links to longer video instructions for a fee, of course. The free stuff is pretty good; I wouldn’t recommend paying for anything as a matter of principle, but that’s just me.