I suspect most people in 2023 read this to get a quick (30 page or so) introduction of the cosmological argument, it’s reasoning, and why it might be tricky. Though I think at the end of that day that’s pretty much where I was coming from too, I think I didn’t quite know what I was going to read and instead found myself most impressed by the Greek influence and parallels with Augustine. Theology, specifically medieval catholic theology, is not something I’ve read too much of and when I have it’s been of a more modern post-reformation variety, so for most of the book I enjoyed finding parallels to things I’m more familiar with (Plato, Aristotle).
The text is brief, but I really think it shows some of the highs and lows of philosophy, which I think is super interesting. The first high begins right in the Prologue

I began to wonder, when I considered that it is constructed out of a chaining together of many arguments, whether it might be possible to find a single argument that needed nothing but itself alone for proof, that would be itself be enough to show that God really exists

This seems a perfect example of how a little creativity, curiosity, and willingness to try can produce so much. The more I learn it seems like the questions that gnaw at us from young ages serve as a loose string on a ball of yarn. One is most rewarded by pulling on said string (although always with caution).
Getting into the actual text there’s a very Augustine-esk confession chapter at the front with tidbits like “How wretched human being are! They have lost the very thing for which they were created.” And Anselm shows himself true medieval with more references to angels and old ages that are harsh on our modern ears. There’s also some talk about essence that seems pretty Aristotelian. The second chapter introduces the main proof which is introduced then ends like this:

And surely that than which a greater cannot be thought cannot exist only in the understanding. For if it exists only in the understanding, it can be thought to exist in reality as well, which is greater.

Here I have in my notes “Reminds me of the Plato dialogue where he talks about thinking about nothing.” A lot of this seems obscure at first, primarily because Anselm uses the phrase “that than which a greater cannot be thought” as a noun, but it actually reads much cleaner than it could, although it seems Anselm doesn’t bring the same level of rigor to bat that someone like Augustine or Aquinas does. His argument, which, as mentioned before, really doesn’t even sound remotely plausible to modern ears, ultimately breaks down.
The second part of the writing is a copy of a letter from another reader telling him in good faith that he must be wrong. Some of the points why he’s wrong go as follows:

1. To rely on the nature of an essence (here its having to exist) also relies on the essence to exist in the first place (fallacy)
2. If thinking about the essence someone how touches the essence itself, then thinking about unreal things would be like touching the essence and you could very easily think about an essence which requires existence too.

This refute is another one of the highlights. It’s cool to see discourse happening which seems unclouded by religious judgement or agenda. Gaunilo’s reply seems very raw, like just another person who wants to get to the bottom of things. A lowlight, however, is Anselm’s response. He clearly doesn’t buy Gaunilo’s issues and doubles down on his answers. But perhaps lowlight is too strong. Anselm makes some pretty good points and it’s pretty interesting all around. I think the main issue here is that it’s troubling to see someone who is smart refuse to come to terms with what seems to the reader the obvious better argument. It almost is a little embarrassing and makes you want to avoid being wrong like Anselm.
The rest of the pages in the first part Anselm goes through a number of other questions that he asks God, appealing in good faith to have him help guide the way. I can’t imagine what a confession written out with medieval calligraphy feels like, but I imagine this is about peak 11th century revelation here. Some of my favorite excerpts are:
Anselm:

• “But how do you spare the wicked if you are completely and supremely just?”
• “But clearly, you are whatever you are, not through anything else, but through yourself.”
• “Why does my soul not perceive you, O Lord God, if it has found you?”
• “That God is greater than can be thought”
• “you are in an unqualified sense”
• “Whatever can be thought to exist but does not in fact exist, can be though of as beginning to exist.”

Gaunilo:

• “But in the case of God, I can think of him only on the basis of the word; and one can seldom or never think of any true thing solely on the basis of a word.”
• “So much, then, for the claim that that supreme nature already exists in my understanding.”
• “I know with absolute certainty that I myself exist, but nonetheless I also know that I can fail to exist.”