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I was doing some exercises and the book that I'm using translated:

> I don't want shoes

to

> 靴は欲しくないです

I answered using が particle instead は, since, as far as I know, I should use が
before 欲しい. So why did the book use は this time?

I imagined that the reason is to make a contrast/comparison that は provide. Is
that why?
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は-and-が
Using は before 欲しい instead が
He may be asked “what do you want as a present?” or “ do you want shoes as a
present?”  
Then he uses は containing contrast/comparison meaning.  
He wants something but shoes.

靴は欲しいです。  
I want shoes. (He wants other things too.)  
靴が欲しいです。  
I want shoes. (He doesn’t want other things.)  
靴が欲しくないです。  
I don’t want shoes.(He’ll accept anything but shoes.)  
靴は欲しくないです。  
I don’t want shoes.(He’ll refuse something besides shoes.)
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_**" I want xxx." vs. "I do not want xxx."**_

When you want something, you will generally say:

> 「XXX + **が** + ほしい」

and when you do not want something, you will generally say:

> 「XXX + **は** + ほしくない」

**The only times you can and must use** 「XXX + **が** + ほしくない」 **is when it is
used in the if-clause or relative clause.** Thus, it is correct to say:

> Relative Clause:「靴{くつ} + **が** + 欲{ほ}しくない人は靴屋{くつや}へは行かない。」
>
> "A person who does not want shoes would not go to a shoestore."
>
> If-Clause:「靴 + **が** + 欲しくなければ、なんで靴屋に来たの？」
>
> "If you do not want shoes, why did you come to a shoestore?"

Moving on...

_**The contrastive 「は」**_

This is essentially a different topic from what has been discussed above.

It is correct to say:

> 「靴 + **は** + 欲しくない。」

when you do not want shoes but want another thing. Thus, you can say:

> 「靴 **は** 欲しくないけど、ジーンズ **は** 欲しい。」

That is a pair of contrastive は's. "I don't want shoes, but I want some
jeans."

> I imagined that the reason is to make a contrast/comparison that は provides.
> Is that why?

No, as I stated above. We are talking about two different things. One is
affirmative vs. negative. The other is contrastive.
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As to the question of when to use はほしい [はほし **い** and not ほし **くない** ]:

I think that @l'électeur gave one good example (=when emphasizing the contrast
of what one wants and what not) with the example 「靴は欲しくないけど、ジーンズは欲しい。」

IF YOU READ FURTHER [NOT ADVISABLE, BUT I DON'T DELETE THIS

  1. Due to the valuable comments

  2. To indicate when **は** 欲しい IS **NOT** USED ;-) 

  3. To call for additional case where **は** 欲し **い** (and not e.g. が欲しい or は欲しくない) is used

]

In addition I can think of at least two other situations to use は but at least
I clicked on this question to find out more, so I hope the natives / gurus
[also others than @l'électeur ] correct / add:

Can be used like in a [phrase or at least close] "あれはほしいこれはほしい" as a critical
statement about someone with too many requests [and here it is more about
requests than aspirations], i.e. in a stereotypical case two employees could
talk about their boss [or similarly 2 managers could talk about someone's
difficult team member] who is never satisfied and comes up with continues and
unreasonable requests.

Can be used when the target of the aspirations is a bit abstract and/or not
**immediately** achievable / obtainable, like I want that kind of life
[ああ言う人生はほしい]
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