I have a nice white-paper on transformer design written by R.G. Keen where he describes all of the important parameters involved in judging the quality of the trans/how they are constructed/how to test and quantify them and how to reverse engineer them. It is very easy to follow yet it is not so simple that you get bored reading it. I would enjoy discussing the art of trans design if anyone was interested. For instance how important is it to keep the leakage inductance as low as possible?