A good book dedicated to aiming is “Answers to a Pool Player’s Prayers”. It is short and only covers the aspect of aiming, no stroke, no follow through, just aiming. It covers true eye, dominate eye and center eye and what happens to each depending on your facial alignment to the cue stick. Everything makes sense but most people do not think about it. I was just starting down that road when I got the book so it really accelerated my aiming progress. He explains (and makes sense) why left or right dominant eye can cause inconsistencies.
To make sure you have your head aligned vertically and not crooked you can use a mirror that is at least the size of a sheet of paper. Tape a string vertical, going up and down using a T-square or some other device to make sure the string will be perpendicular to the table, not slanting toward one side or the other.
Set the mirror up on the table. Place the CB a few inches away from it and inline with the string so the string is in the center of the CB. Pretend to shoot the CB into the image on the mirror. After you are lined up and ready to shoot look at your face and cue in the mirror. Is the cue in a straight line with the string? Where does the string intersect you face? Is it straight down the middle? If so you are in a centered aim position, which has a lot of advantages. Is it directly through the middle of the left eye? If so you have true left eye alignment which also has advantages. Is it halfway between the left eye and center of the face? Then you have dominant left eye aiming.
None of these aim alignments are wrong but they have their advantages depending on what focus you use along with the aim technique. Also, is your head aligned vertically with the string or is it on a slant. If on a slant it makes aiming more difficult because each eye is seeing a different view and your brain has to make sense out of it. The dominant eye also has disadvantages because one time you may shoot at 80% – 20% then the next time you may be 70% – 30%. The change makes for a different view with respect to parallax and it may look good but is off. It is hard to be consistent with dominant eye. To see what I mean, just aim a shot as normal then without moving shut one eye and look at you aim. It looks different, like it is going to miss. Then shut the other eye and it will look different in the other direction. Your brain figures all this out but if you keep changing alignment its like shooting a gun with the site being changed before you aim.