I’ll reinforce what KPChief said and add a bit. There is a theoretical and a practical aspect to investigation, where the theory argues how it should be done and the practical documents how it is done.
For the theoretical stuff, I definitely recommend reading Prof. Nancy Leveson’s stuff. A slightly earlier draft of her book is available free on her site:
As well as a ton of other material. Coming from (I assume) a maritime background I would especially recommend one of her students’ analysis of the Sewol-ho ferry accident in South Korea. It’s a gut-wrenching read:
For the practical side, I’d recommend picking an accident of interest and going to the NTSB docket site:
and searching for the docket. Don’t just read the final report: go through the docket and read the intermediate reports and the testimony. Then decide how you would have put together the final report.