I've spent way too much time comparing the 'Baldur's Gate' novel by Philip Athans to the original game, and honestly, the connection is pretty loose. It's less a direct prequel and more of a companion piece that uses the same setting and a couple of major characters, but tells its own story. You follow Abdel Adrian, who is supposed to be the canon version of the player character from the game, but his portrayal is wildly different—way more of a brutish, grumpy mercenary than the blank slate you project yourself onto. The novel covers events leading up to the game, like the iron crisis and Sarevok's plot, but it condenses and changes so much.
The biggest link is seeing familiar faces like Imoen, Gorion, and Elminster, but they often feel off-model. Imoen is suddenly Abdel's sister? That was a huge departure that never sat right with me. The book rushes through plot points the game lets you explore over dozens of hours, so it lacks the depth and player agency. If you go in expecting a faithful novelization, you'll be disappointed. It's more useful as a curiosity, a look at what the 'official' canon was trying to be in the early 2000s, before later games and lore kind of soft-retconned a lot of it. I keep it on my shelf mostly for nostalgia and as a weird piece of Forgotten Realms history.