;;; .dir-locals.el

((nil
  (eval add-hook 'find-file-hook (lambda ()
                                   (message "my find-file hook called...")) nil t)))

;;; .dir-locals.el

((nil
  (eval add-hook 'find-file-hook (lambda ()
                                   (message "my find-file hook called...")) nil t)))

The nil means this applies to all files in the directories bellow .dir-locals.el.

The t passed to add-hook makes find-file-hook buffer-local. Therefore the hook runs only for this file.

dir-locals runs before find-file-hook, so the whole thing works.