Missed

Missed
 

p.393
-[...] missed spells hit books on shelves [...]

Why missed but not missing?(=those spells missing things)

These were spells that missed the intended target, because the wands were pointing in all directions.
These were 'misguided spells'.

Note that the preterit tense is in use:
Wands flew in all directions, spells missed their target, hitting books instead and sent them flying.

'missing spells' = spells that have been lost/omitted/taken

'misguided spells' = spells that hit unintentional targets

The trajectory of the spell went amiss/awry/askew. 'Misguided spells hit books on shelves.'

Thank you for the answer. But this is the point:

A shark that eats men is a man-eating shark.
A man that dances is a dancing man.

Then wouldn't it be

A spell that misses is a missing spell.

?

I believe your answer is good, but I am slow on the uptake, so I hoep you don't mind explaining a bit more, even though I know it could be a tough job in explaining language by using language.

Sorry, but there isn't much more that I could explain.

A 'missing spell' is not the same as a 'missed spell'. You simply cannot use 'missing spell', as you would think. It doesn't make sense.
And the original sentence is in the past tense. You can't throw 'missing' in the mix.

In order to have 'missing' in the sentence, you'd have to change the tense everywhere. And 'missing spells' will still not make sense.

Wands were flying in all directions, misguided spells were hitting books ...

Thank you for the excellent answers. I think I need more time to think about it.

Every language has collocations - words that are usually used together in a certain way - and we object to alternate wording.
For example, we commit a crime, do a jig, make a decision. We can't switch the verbs around. We can't 'commit' or 'do' a decision. We can't 'do' or 'make' a crime. We can't 'commit' a dance.

Glasses are described as crystal clear when they are clean. But clean clothes can never be crystal clear.

The words 'quick' and 'fast' are considered synonyms, but we don't always use them interchangeably. It's natural English to talk about taking a quick shower, for example. It's not natural to hear someone say they're taking a fast shower. We eat fast food, not quick food.

By the same token, a missing spell is not a misguided spell.
A missing spell is a spell that can't be found.

In fact, a 'missed spell' is a collocation I don't recognize, while 'misguided spell' is.
If you were to Google 'missed spell', you'll find perhaps one or two cases where it is used to mean a misguided spell. (Unfortunately, most of the search results come from folks who don't know how to spell 'misspell'.) If you search for 'missing spell', on the other hand, you'll see that it always means a spell that can't be found. It's missing.

I agree with Randy: the adjective "missing" means only "lacking", "absent" or "not found". It does NOT mean "failing to hit (some target)".

Thank you, I guess I'll just have to memorise special expressions.

p.135
-If Malfoy wants something fixing, [...]

Why fixing not fixed? Is it a typo?

It looks wrong, but context may come into play.

For instance: Malfoy wants something, fixing his eyes on me with an intense stare.

Good answer. Thank you.

It looks wrong to American English speakers, but it is good British English.
We Americans would say 'fixed'.

Good answer. Thank you.