In Minecraft 1.3, the maximum enchantment level has lowered from 50 to 30. With this change, they also lowered the number of bookshelves needed to achieve the maximum enchantment level, it is now 15 instead of 30 bookshelves.
There seems to be a lot of confusion when people try to get a specific enchantment level. I have some tips that should help you out when trying to figure this out.
The first thing you need to do is if you still have 30 bookshelves, remove half of them. Having all 30 causes you more trouble than it is worth, and gives you no benefit.
Next you need to understand how each bookshelf affects the maximum enchantment level you can get from the enchanting table. Below is a list of the number of bookshelves surrounding your enchantment table followed by the possible enchantment levels:
Important Note: When using between 0 and 4 bookshelves the highest level is not always listed last, but between 5 and 15 bookshelves, the highest level is always last.
0 - Levels 1 - 8
1 - Levels 1 - 9
2 - Levels 1 - 11
3 - Levels 1 - 12
4 - Levels 1 - 14
5 - Levels 1 - 15 (10 occurs most often)
6 - Levels 1 - 17 (12 occurs most often)
7 - Levels 1 - 18 (14 occurs most often)
8 - Levels 1 - 20 (16 occurs most often)
9 - Levels 1 - 21 (18 occurs most often)
10 - Levels 1 - 22 (20 occurs most often)
11 - Levels 1 - 24 (22 occurs most often)
12 - Levels 1 - 26 (24 occurs most often)
13 - Levels 1 - 27 (26 occurs most often)
14 - Levels 1 - 29 (28 occurs most often)
15 - Levels 1 - 30 (30 is always the 3rd option)
As you can see, even though an enchantment level is possible with a certain number of bookshelves, it may be more easily obtained using a higher number of them. Example: If you want level 24, you can get this with 11 bookshelves. However, it might take a bit of time to get it since level 22 happens more often. In this case you would want to use 12 bookshelves and will most likely get level 24 on your first try.
I hoped this helped a little bit with understanding the new enchantment levels.