5. 统计Python之禅（如下文字）中英文字母出现的频率，按照字母出现频率的降序进行排列，并将字母及其出现频率以csv格式保存在‘word-frequency.csv’文件中。

在Python中，我们可以使用`collections`模块的`Counter`类来统计字符串中每个字符的出现次数，然后结合`pandas`库对数据进行排序并保存到CSV文件。首先，你需要安装`pandas`，如果还没有安装可以使用下面的命令：

pip install pandas

下面是实现这个任务的步骤：

import collections
import re
import pandas as pd

# Python之禅内容
zen = """
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never perhaps sometimes is better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
"""

# 将文本转换为小写并移除非字母字符
cleaned_text = re.sub(r'\W+', '', zen.lower())

# 使用Counter统计字母频率
letter_counter = collections.Counter(cleaned_text)

# 转换为DataFrame以便排序
df = pd.DataFrame.from_dict(letter_counter.items(), orient='index', columns=['Frequency'])

# 按照频率降序排列
df = df.sort_values(by='Frequency', ascending=False)

# 保存到CSV文件
df.to_csv('word-frequency.csv')

完成以上代码后，`word-frequency.csv`文件将会包含Python之禅中字母及其出现频率的统计结果，按照频率从高到低排列。