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What is the additional nuance in meaning when の is inserted before なら? For
example:

> トムが行く(の)なら、私も行きます。

In the Tobira Gateway to Advanced Japanese (p. 69) it says

> の occurs before なら when the supposition is based on what the speaker has
> heard from someone or learned from the situation.

However, other websites such as <https://maggiesensei.com/2015/11/07/how-to-
use-%E3%81%AA%E3%82%89-nara/> and <https://jlptsensei.com/learn-japanese-
grammar/%E3%81%AA%E3%82%89-nara-meaning/> explain that の adds emphasis.
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question
meaning particles
のなら: emphasis or hearsay?
This の is an [explanatory-の](https://japanese.stackexchange.com/q/5398/5010).
Roughly speaking, it adds the nuance of "(if) ... is the case" or "(if) it's
that ...". This の is usually optional, but it indicates the conditional is
something that does matter now.

You can use の to seek clarification in an interrogative sentence (see
[this](https://www.wasabi-jpn.com/japanese-grammar/question-markers/)). This
connotation is preserved when it's used with なら. If you understand the
difference between "行く？" and "行くの？", you know the difference between 行くなら and
行くのなら, too. This type of の is often used with external information, but の is
not a hearsay marker _per se_. It's not a plain emphasis marker that turns "if
Tom comes" to "if Tom does come", either.

With that said, it's okay to add の unconditionally most of the time in
conversations. The difference is usually very small. の tends not to be used
with "generic fact" or "purely hypothetical" conditionals that have little to
do with the current situation at hand (e.g., コインを投げて表が出るなら, 明日地球がなくなるなら,
サンタクロースが実在するなら, ...).
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answer
The biggest difference for me is that の makes it a bit harder to pronounce
when I speak and sticks out a bit in my head when I hear it. This difference
goes away when の is reduced to ん.

I would say the following two sentences are practically interchangeable. I
feel no particular emphasis in one over the other.

> トムが行くなら、私も行きます。
>
> トムが行くんなら、私も行きます。

なら could be rephrased using ば but の is mandatory in this case.

> x トムが行けば、私も行きます。
>
> トムが行くの／んであれば、私も行きます。

I understand this as meaning the function of の is already included in なら. You
are already assuming a particular situation and describing that situation as
you would when you use の.

* * *

[Edit]

If I had to nitpick a difference, I would say the version with の gives an
impression that the described situation is already decided outside of the
speaker’s control, as if to say:

> トムが行く（ことになっている）のなら、私も行きます。  
>  If (it is already decided that) Tom goes, I will go, too.

The textbook is probably referring to this feeling of the information coming
from an external source, as opposed to the speaker coming up with their own
assumption.

But again, you could say the same thing without の.

> トムが行くことになっているなら、私も行きます。

So I would say the difference is mostly negligible.
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answer