Make the persp-mode save|restore nov.el EPUB buffer. Tested in my spacemacs.

persp-nov.el

(with-eval-after-load "persp-mode"
  (with-eval-after-load "nov"
    (add-hook 'persp-save-buffer-functions
              '(lambda (b)
                 (when (eq 'nov-mode (buffer-local-value 'major-mode b))
                   `(def-nov-buffer ,(buffer-name b)
                      ,(buffer-local-value 'nov-file-name b)
                      ,(buffer-local-value 'major-mode b)))))
    
    (add-hook 'persp-load-buffer-functions
              '(lambda (savelist)
                 (when (eq (car savelist) 'def-nov-buffer)
                   (with-current-buffer (get-buffer-create (cadr savelist))
                     (setq buffer-file-name (expand-file-name (caddr savelist)))
                     (require 'nov)
                     (nov-mode)))))))