Speed up COUNT(*) when using an index
The query will be slow because cardinality of category index is low. There are 12 categories, so in average the query will read 1/12 part of the index. You can't improve this query.

Your original approach can improve overall performance. Just instead of manually updating book_count create a trigger on INSERT and DELETE event.

UPDATE: To prove the query will partially read index category

mysql&gt; select count(*) from books;
+----------+
| count(*) |
+----------+
|     1000 |
+----------+
1 row in set (0.00 sec)

mysql&gt; select category, count(*) from books group by 1;
+----------+----------+
| category | count(*) |
+----------+----------+
|        0 |       50 |
|        1 |       77 |
|        2 |       88 |
|        3 |       84 |
|        4 |      102 |
|        5 |       79 |
|        6 |       79 |
|        7 |       73 |
|        8 |       84 |
|        9 |       76 |
|       10 |       87 |
|       11 |       83 |
|       12 |       38 |
+----------+----------+
13 rows in set (0.01 sec)

mysql&gt; flush status;
Query OK, 0 rows affected (0.00 sec)

mysql&gt; select count(*) from books where category = 6;
+----------+
| count(*) |
+----------+
|       79 |
+----------+
1 row in set (0.00 sec)

mysql&gt; show status like 'Hand%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Handler_commit             | 1     |
| Handler_delete             | 0     |
| Handler_discover           | 0     |
| Handler_external_lock      | 2     |
| Handler_mrr_init           | 0     |
| Handler_prepare            | 0     |
| Handler_read_first         | 0     |
| Handler_read_key           | 1     |
| Handler_read_last          | 0     |
| Handler_read_next          | 79    |
| Handler_read_prev          | 0     |
| Handler_read_rnd           | 0     |
| Handler_read_rnd_next      | 0     |
| Handler_rollback           | 0     |
| Handler_savepoint          | 0     |
| Handler_savepoint_rollback | 0     |
| Handler_update             | 0     |
| Handler_write              | 0     |
+----------------------------+-------+
18 rows in set (0.01 sec)