added activebook reducer

book_list/src/reducers/index.js

@@ -1,8 +1,10 @@
 import { combineReducers } from 'redux';
 import BooksReducer from './reducer_books';
+import ActiveBook from './reducer_active_book';
 
 const rootReducer = combineReducers({
-  books: BooksReducer
+  books: BooksReducer,
+  activeBook: ActiveBook
 });
 
 export default rootReducer;

book_list/src/reducers/reducer_active_book.js

@@ -0,0 +1,10 @@
+// State argument is not application state, only the state
+// this reducer is responsible for
+export default function(state = null, action) {
+  switch(action.type) {
+  case 'BOOK_SELECTED':
+    return action.payload;
+  }
+
+  return state;
+}