I’ve been looking for a Sengoku Japan book but I have seen criticisms of Stephen Turnbull, the only person who seems to come up on searches, when it comes to this area as he does not engage well with sources etc. Can anyone recommend a well researched book on the Sengoku period book with good source engagement?
You're in luck. AH maintains a booklist that offers recommendations on pretty much all major periods and topics, and one part of that focuses on the
Kamakura to Sengoku eras (1192 - c. 1600)
Hopefully that will get you underway with some solid suggestions for further study that will match your interests and reading level.