关于work like a librarian和work as a librarian

各位NN，我有个可能很愚蠢的问题。

在OG11的p632里，为了解释as和like的差别，文中举了两个例子

Correct: "I work as a libarian." Incorrect: "I work like a libarian"

不是说like后面跟名词而as后面跟主谓结构吗，为什么这里名词libarian前面用的是as而不是like呢？

我的理解是因为原文的意思不是想表达“我像图书管理员一样工作”而是“我的工作是图书管理员”。如果说上下文想表达的是“我像图书管理员一样工作”，那就应该说“I work like a libarian”。就好像我们经常在公司说“sb works like a horse”不知道我的理解正确吗？

望指教，非常感谢！

like+n.："象“

as+n.："作为“

like and as (Similarity, function)

We can used like or as to say that things are similar.

1.  Similarity: Like (preposition)
Like  is similar to a preposition.  We use it before a noun or pronoun.
like + noun/pronoun
You look like your sister.  (Not .... as your sister.)
He rain like the wind.
Like his brother, he is a vegetarian.
She's dressed just like me.
We can use very , quite  and other adverbs of degree to modify like.
He's very like his father.      She looks a bit like Queen Victoria.
We can use like  to give examples.
She's good at scientific subjects, like mathematics.  (Not ... as mathematics.)
In mountainous countries, like Peru, ....

2. Similarity: as (conjunction)
As  is a conjunction.  We use it before a clause, and before an expression beginning with a preposition.
as + clause
as + prepostion phrase.
Nobody knows her as I do.
We often drink tea with the meal, as they do in China.
In 1939, as in 1914, everybody seemed to want war.
On Friday, as on Tuesday, the meeting will be at 8: 00am.

3. as  with inverted word order.
In a very formal style, as  is sometimes followed by auxiliary verb +subject
She was a Catholic, as were most of her firends.
He believed, as did all his family, that money is everything.

4. as you know etc
Some expressions beginning with as are used to introduce facts which are 'common ground' -- known to both speaker/writer and listener/reader.  Examples are as you know , as we agreed, as you suggested.
As you know, next Tuesday's meeting has been cancelled.
I am sending you the bill for the repairs, as we agreed.
There are some passive expressions of this kind -- for example as is well known; as was agreed.  Note that there is  no subject it   after as   in these expressions.
As is well known, more people get colds in wet weather.  (NOT As it is well known....)
I am sending you the bill, as was agreed.  (NOT....as it was agreed.)

5.   comparison with as  and  like  after negatives:
After a negative clause, a comparison with as or like usually refers only to the positive part.
I don't smoke, like Mary.  (Mary smokes.)
I am not a Conservative, like Mary.  (Mary is a Conservbative.)
Before a negative clause, the comparison refers to the whole clause.
Like Mary, I don't smoke. (Mary doesn't somke.)

6.   function or role: as  used as a preposition.
Another use of as  is to say what function or role a person or thing has -- what jobs people do, what purposes things are used for, what category they belong to, etc.  In this case, as  is used like a preposition, before a noun.
He worked as a manager for two years.  (not ... like a manager)
Please don't use that knife as a screwdriver.
A crocodile starts life as an egg.

compare this use of as with like.
As your brother, I must warn you to be careful. (i'm your brother.)
Like your brother, I must warn you to be careful. (i'm not your brother, but he and I have similar attitudes.)