Is it necessary to use the past perfect if a time expression makes the order of events clear?

On the Michael Vince’s book, English Grammar and Vocabulary, there is this sentence: ” Mallory had discovered an ancient map showing the position of the city, although no European had ever been to the area before.”

Why can’t it be: “ Mallory discovered an ancient map showing the position of the city, although no European had ever been (...).” ?

According to various online sources, the alternative you have given is acceptable also. I feel the question you have is different from the subject of the example sentence you gave, but I'll try and address both topics. One of the uses of the past perfect is to refer to an event in the past that happened before another event in the past. Here are some examples given on the British Council grammar website:

Look at these 2 sentences.

• James had cooked breakfast when we got up.
  
• James cooked breakfast when we got up.

In the first sentence, the past perfect tells us that James cooked breakfast before we got up. In the second sentence, first we got up and then James cooked breakfast.

and

Look at some more examples of the past perfect.

• When Mrs Brown opened the washing machine she realised she had washed the cat.
  
• I got a letter from Jim last week. We’d been at school together but we’d lost touch with each other.

The past perfect is used because they were at school before he received the letter. It refers to an earlier past.
learnenglish.britishcouncil.org

In the example you found:

Mallory had discovered an ancient map showing the position of the city, although no European had ever been to the area before.

The past perfect that's important in the function I mentioned above is the second use of it:

", although no European had ever been to the area before."

This is because the past perfect can be used for the event that comes earlier than something else that's already in the past. In this case, Europeans having been to the area before is the event referred to that comes earlier, specifically earlier than when Mallory "found" or "had found the map".
Based on the grammar recommendations I found online your proposed sentence should be grammatical also.

Some more examples from grammarly.com:

When you’re talking about some point in the past and want to reference an event that happened even earlier, using the past perfect allows you to convey the sequence of the events. It’s also clearer and more specific. Consider the difference between these two sentences:

• We were relieved that Tootles used washable paint.
  
• We were relieved that Tootles had used washable paint.
  

grammarly.com - past perfect

Based on the numerous sources I've checked the past perfect in:

"Mallory had discovered an ancient map..."

Is not strictly necessarily, unless this event itself is referring to a point in time with reference to another point in time in the past that came after it.

The same description of past perfect is given at the Wikipedia article for it. It explains it is:

... used to refer to an action at a time earlier than a time in the past already referred to...
Wikipedia - pluperfect (past perfect)

Having the earlier event (verb) in the past perfect ("Mallory had discovered an ancient map..." instead of the simple past ("Mallory discovered an ancient map") may be for the reason I suggested earlier, that it's an even earlier event than another past event, or may be just done for a stylistic reason, to preserve parallelism, that is, to have "Mallory had discovered..." and "no European had ever been ..." both in the past perfect. I'm not exactly sure which of the two it is.

The Wikipedia article on the past perfect gives this example:

A man who for years had thought he had reached the absolute limit of all possible suffering now found that suffering had no limits, and that he could suffer still more, and more intensely.
Wikipedia - Past perfect

The article explains "had thought" and "had reached" are both in the past perfect because they are events both things that happened before another thing, namely that thing being a man's realization about suffering.

So in my opinion, your sentence could easily read as "Mallory discovered an ancient map...". Again, it's important to consider whether the discovery of the map isn't itself with reference to the event occurring even earlier in time than another event in the past, either explicit or implied.

Also, although the past perfect is described in the way I mentioned above, that is a past event occurring before another past event, it's entirely possible to hear it used outside this context and to be used interchangeably (in my experience), at least colloquially, for the simple past. I may say "I had gotten angry so many times" instead "I got angry so many times", and although I'm not sure if it's 100% correct, given that the past perfect is described as I did so above, it's something I would assert can be heard frequently enough in casual speech, even though it may not meet the criteria as outlined in grammar guides about it reference needing to be a past event before another past event.

Finally, I feel your intention for asking this question is actually based on something different from what the example you gave illustrates. I think what you meant to ask was something like if the following was grammatical:

Mallory discovered an ancient map showing the position of the city, although no European was ever in the area before the map's discovery.

I think you want to know whether it's necessary to use the past perfect, ie., "although no European had ever been in the area before the map's discovery" instead of the simple past "although no European was ever in the area before the map's discovery." That question is a good one, and I don't know whether it's absolutely required, given that the sentence gives enough information about the time sequence of events. Grammar guides say the past perfect is used for this purpose, but don't specifically say you "must" use it, and if the sentence is clear using the simple past instead of the past perfect, I'm just not sure whether you "must" use it or not. I'm interested in finding the answer to that, or even if there's a consensus. Informally I don't see much wrong with:

I thought I won that point before the referee ruled it out of bounds.

instead of:

I thought I had won that point before the referee ruled it out of bounds.

However, in my opinion, the past perfect is preferable. Whether it's required, I don't know.