: Why would a novelist choose to use the present tense? I'm currently reading The Crossing Place by Ellie Griffiths and a couple of pages in I realised that the entire book is in the present

Many reasons can drive the choice of the present tense and, personally, I often use it. Note that you can switch from past tense to present, and back: you are not tied to one or the other, so the reasons to use the present tense may be local.

The first reason that came to my mind is that present tense is more quickly immersive. When you start your story using past tense, the reader should expect some background information. Using the present, you can place the reader directly in the action, delaying explanations for later. So you can raise the suspense very effectively, for example.

A second reason is not to give any clue about the end. Suppose you choose the first person point of view, which is very immersive. If you choose to use past tense, then the reader will expect the character to survive at the end of the story, which is not the case with present tense.
Compare:

I did not know if I had a chance to reach the surface and take another breath.

and

Will I reach the surface and take another breath?

A third reason is to make the temporal composition of the story clearer. Many events in the past may be considered to explain the situation of the story. You may have one past event, then another one taking place before, and then another taking place between them, and so on. It may be very complicated and hard to manage. Grammar has some resources to help, placing one temporal line to the present time will give a clear reference point for the other lines.

A fourth reason: when the reader shares the narrator emotions, the feelings will not be the same. If the events take place in the past, the reader will have the feeling of a remembrance of an emotion, not the emotion itself.

For example:

I was afraid. The giant squid was swimming quickly after me, and near to caught me.

and

I'm scared. The giant squid is swimming fast. It will catch me!

All my examples deal with suspense and fear, cases where the need to use present tense is obvious. But it is also true for any other emotions or feelings.

I think there may be other reasons... I'm unable to think about more at the present time.

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It can be a device to breathe more life into a story, in particular to evoke the feeling of witnessing events or action. Note that if you are told in the past tense, then the narrator has hindsight and could theoretically be viewing the past through more recent events; looking with kindly eyes on someone who was previously mean but would subsequently improve. However in the present tense you cannot be given that kind of information; you can only know what the narrator knows at that point in time.
It needn't be jarring; as with any art the best way to receive from it is to just experience it. See this, perhaps, as a more cinematic experience than a past-tense novel would be (as discussed here by Mignon "Grammar Girl" Fogarty).
Let's compare a scene:

Misha sat down on the sofa and looked over at me. She smiled a small
smile, but I could read the pain in the redness of her eyes.
Misha sits down on the sofa and looks over at me. She smiles a small
smile, but her eyes are red, and I can read pain in them.

In the present tense we have to largely experience things in the order that the narrator does, and the thoughts in their order too. We are not treated to analysis or retrospect, we get a stream of events.
It is a device for the writer to convey immediacy, and perhaps to add an edge of being present in the moment, rather than of reflection, and can help the writer (and the reader) focus on the experiential aspect of a scene rather than the symbolism or the abstract. As with any technique, it can succeed or it can fail.
I look over at the tree I can see out of the window, over the rooftops. It is utterly still, until a breeze riffles some leaves, and a bird shoots up from within the tree. I feel trapped at my desk, in the intense corporate stillness; I want the freedom to ascend.

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