One of my textbooks said that you cannot use the particle は with 何 and
undefined words of the こそあど.

Yet I'm sure I've heard これは何ですか。 many times before, and これが何ですか。 sounds
unnatural to me.

What is the correct usage?
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question
particles は-and-が kosoado
は and が with 何 & こそあど
In most cases, が is used before a question word (何、どれ、etc.) as opposed to は.
This is because が has the nuance that it emphasizes this word which is the
same reason が is usually used when answering a question (ex: "誰がやったの？" =>
"先生がやったよ")

Depending on the nuance trying to be conveyed, が or は can be used naturally
after any non-question こそあど words.
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answer
The key word is "undefined". は is a topic marker, and you can't switch topics
to something you don't know about yet.

In 「これは何ですか？」, は is acceptable because これ is a **specific, definite thing**
(at least in context) and 何 is about asking for more information about that
thing.

In「何が好きですか？」. the 何 isn't "definite" to the speaker because it is a
"placeholder for anything" 何. Of course, 好き puts an additional constraint on
it, and if you re-arrange the sentence, you can say 「好きなものは何ですか」 because 好きなもの
is definite enough to switch topics to with は.

**On a side note** , が is the subject marker. Every complete phrase must have
a subject, so you might wonder why sometimes there isn't a が. Sometimes it is
because the subject is implicit... but sometimes it is because the subject is
the same as the topic.

For example, in 「私はアメリカ人です」, 私 is both the topic and the subject. That is, the
actual meaning is (note: this is not correct Japanese for basic subject-
marking use of が!) 「私はがアメリカ人です」. But Japanese has a forced grammatical
convention where the は and が particles collapse to just は when the same noun
is being marked as both subject and topic (except in certain other special
usages of が). In other words, が MUST be abbreviated out in these cases. This
is consistent with the view presented on p.118 of "A Dictionary Of Basic
Japanese Grammar".
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answer