There are a few texts devoted strictly to transformers in power circuit design, however they are largely concentrated on the area of SMPS and Inverter design. A quick search on Amazon for the keywords, "Transformer Electronics" should bring those up for review.
National Semiconductor published a text back in the seventies, my college years, which covers many aspects of DC regulator design, of course utilizing mainly their components at the time:
National Voltage Regulator Handbook
A few noteworthy out-of-print texts worth having on the bookshelf, which devote a chapter or more on various aspects of the topic are as follows, alas little on transformers:
Linear Integrated Circuits by Jack Winzer
Winzer's text devotes two chapters to the theory and design of linear regulator circuits with many step-by-step detailed design examples as well as covering many other useful topics throughout the remainder of the text.
Electronic Devices and Circuits by David A. Bell
Operational Amplifiers: Applications, Troubleshooting, and Design by David A. Bell
Bell's texts are some of the most under appreciated, well written, concise and thorough texts covering their respective topics I have read hands down. The first text, Electronic Devices and Circuits, 4th Ed, devotes two or more chapters to the topic, while the second text devotes only a single chapter on the designs incorporating op-amps, Operational Amplifiers: Applications, Troubleshooting, and Design, however it is certainly the most detailed while approachable texts covering virtually every facet of designing with op-amps. Both texts utilize actual step-by-step detailed design examples including where and how to find those critical specifications from datasheets.
As the above four texts are out-of-print, they can be purchased for a mere fraction of those texts still in print and utilized in today's classrooms.
BigDog