diff --git a/fftcg/book.py b/fftcg/book.py
index 1962e2c..277266b 100644
--- a/fftcg/book.py
+++ b/fftcg/book.py
@@ -2,6 +2,7 @@ import logging
 
 from PIL import Image
 
+from .cards import Cards
 from .imageloader import ImageLoader
 
 
@@ -21,7 +22,7 @@ class Book:
     # Card back image by Aurik
     __BACK_URL = "http://cloud-3.steamusercontent.com/ugc/948455238665576576/85063172B8C340602E8D6C783A457122F53F7843/"
 
-    def __init__(self, cards, grid, resolution, language, num_threads):
+    def __init__(self, cards: Cards, grid: tuple, resolution: tuple, language: str, num_threads: int):
         logger = logging.getLogger(__name__)
 
         # sort cards by element, then alphabetically
@@ -46,7 +47,7 @@ class Book:
         # width, height per card
         w, h = resolution
 
-        def paste_image(page, index, image):
+        def paste_image(page: Image, index: int, image: Image):
             x, y = (index % c) * w, (index // c) * h
             page.paste(image, (x, y))
 
@@ -65,6 +66,6 @@ class Book:
 
             self.__pages.append(page)
 
-    def save(self, filename):
+    def save(self, filename: str):
         for i, page in enumerate(self.__pages):
             page.save(filename.format(i))
diff --git a/fftcg/card.py b/fftcg/card.py
index e4feb0e..13b8501 100644
--- a/fftcg/card.py
+++ b/fftcg/card.py
@@ -13,7 +13,7 @@ class Card:
         '闇': "Darkness"
     }
 
-    def __init__(self, data, language="EN"):
+    def __init__(self, data: dict, language: str = "EN"):
         if not data:
             self.__opus = "0"
             self.__serial = "000"
diff --git a/fftcg/imageloader.py b/fftcg/imageloader.py
index e556fb7..ca5ea5b 100644
--- a/fftcg/imageloader.py
+++ b/fftcg/imageloader.py
@@ -8,7 +8,7 @@ from PIL import Image
 
 
 class ImageLoader(threading.Thread):
-    def __init__(self, url_queue, resolution, language):
+    def __init__(self, url_queue: queue.Queue, resolution: tuple, language: str):
         threading.Thread.__init__(self)
 
         self.__queue = url_queue
@@ -44,7 +44,7 @@ class ImageLoader(threading.Thread):
             self.__queue.task_done()
 
     @classmethod
-    def load(cls, urls, resolution, language, num_threads):
+    def load(cls, urls: list, resolution: tuple, language: str, num_threads: int):
         url_queue = queue.Queue()
         for url in urls:
             url_queue.put(url)
diff --git a/fftcg/opus.py b/fftcg/opus.py
index 4802ac2..a965621 100644
--- a/fftcg/opus.py
+++ b/fftcg/opus.py
@@ -6,13 +6,13 @@ from .cards import Cards
 
 
 class Opus(Cards):
-    def __init__(self, opus_id):
+    def __init__(self, opus_id: str):
         logger = logging.getLogger(__name__)
 
-        if isinstance(opus_id, str) and opus_id.isnumeric():
+        if opus_id.isnumeric():
             roman_opus_id = roman.toRoman(int(opus_id))
             params_add = {"set": [f"Opus {roman_opus_id.upper()}"]}
-            self.__number = str(opus_id)
+            self.__number = opus_id
             self.__name = f"opus_{opus_id}"
 
         elif opus_id == "chaos":