It's like trying to find a recipe for cake and finding only cookbook reviews.
But actually a lot of the time, the original sources are the absolute best. In some sense, you can use survivorship bias to your advantage. A book written in the 21st century might be so good that it will be remembered thousands of years from now (the odds are small although nonzero). A book written thousands of years ago, and remembered today, has, by definition, already passed that test.
Jesus himself explicitly addresses this, saying: "I praise You, Father, Lord of heaven and earth, because You have hidden these things from the wise and learned, and revealed them to little children. Yes, Father, for this was well-pleasing in Your sight." (Matthew 11:25-26)
And there are places where Jesus seems to explicitly speak to the reader (Matthew 24:15, "Let the reader understand", spoken orally to a presumably baffled group of disciples with no stenographers present), and even to readers of a far more distant future time than today (Matthew 16:28, "Truly I tell you, some who are standing here will not taste death until they see the Son of Man coming in His kingdom.")
See also Deuteronomy 31:21, "And when many troubles and afflictions have come upon them, this song will testify against them, because it will not be forgotten from the lips of their descendants". God revealed to Moses that even in the far-flung future when people have strayed far from God, the required context will still be printed on every man's heart.
It covers a wide array of training and topics. And you'd think you'd need an encyclopedia of references aside to understand the day and age of which it spoke. It features extensive religious and political references, yet the weird thing is a book written for royalty around 800 years ago feels oddly familiar. And in reality I found a good deal of the advice and knowledge it shared not only interesting but actually quite useful and wise. It also rather gives proof to the meaning of the tale of Damocles, without in any way intending to do such!
[1 - actual book] - http://www.mediumaevum.com/75years/mirror/index.html
[1 - wiki overview] - https://en.wikipedia.org/wiki/Konungs_skuggsj%C3%A1
Stoicism as practiced in antiquity shares a lot with mindfulness and Buddhism. It's about acceptance of the now. And what you can and cannot control. It's not that you aren't allowed to feel, just that you aren't allowed to have your emotion rule your reason.
Bob died. That is sad. You can grieve. But Bob is dead. Nothing is going to change that. Killing yourself so you don't have to live in a world without Bob is not rational. Yelling at someone for asking your a question is not rational. Even if you are sad. You can't control your sadness. But you can control your actions. And your actions should be dictated by reason. Setting up a will that details what happens when you die, that is a rational response to Bob's death. Even if it's informed by sadness. Because his death could have made you realize that you don't have a plan.
But over time, that has been warped into a complete rejection of emotion.
So when we say someone is stoic, that's what we mean, that they're displaying no emotion.
And that's the notion they're arguing against.
My point is what I have found to read about stoicism is either a greatest hits of 200 CE or drawn out rhetoric about the wrongness of the dictionary definition of stoic. In other words, exactly the opposite of your line there, nothing but the shift in meaning.
No one made any mention of why it should be "lack of emotion" rather than the definition from antiquity. It just kind of happened through a multi-generational game of telephone.