There is a serious textbook out there called Southern Hokkien: An Introduction, by Bernhard Fuehrer and Yang Hsiu-fang. I’m not sure how easy it is to buy: there is a pdf (well, three pdfs, one for each volume) floating around online for free, presumably pirated, and its extremely extensive audio should be available too.
It says: We apply a descriptive approach and give priority to the fused dialect most frequently encountered in Taiwan. This dialect variant derives primarily from the Zhangzhou dialect but integrates some characteristics of the dialects associated with Amoy and Quanzhou.
That means it teaches the Zhangzhou-derived dialect preferentially, but identifies the Amoy and Quanzhou alternatives throughout.
It uses a version of POJ romanisation, which is how dialogues are transcribed. But it shows the relevant Chinese character when introducing new vocabulary (and an appendix renders the dialogues into Chinese characters, for reference only).
Grammar explanations generally compare/contrast modern Mandarin usage with Hokkien usage.
The book was a collaboration between the UK’s SOAS university and NTU. NTU has an online course (delivered in Mandarin) teaching the first half of the book here:
It’s a very good academic class, but sadly, like most English classes offered in Taiwan’s public education, I don’t think any of the students will gain fluency through taking this course.
It’s more geared for people who already speak Taigi and learn about the details of the language from the perspective of an other language…
Interesting to know. I’m moving to Taiwan shortly, for a final six months of classroom-based Mandarin study, but have started - and plan to continue - spending a few minutes each day repeatedly listening to and familiarising myself with the dialogues in the book. Six months of that, and I should be ready to start speaking and properly studying Taigi, if I feel like it.