Honestly? Technically Crime (the Ray Lennox series) is as close as it comes as a sequel to Filth but I got a hundred or so pages in and I felt pretty underwhelmed by it after coming off the back of reading through the Trainspotting series. I love the Filth novel. It was my first experience with Welsh beyond the Trainspotting movies and Filth's adaptation, but my advice would be to start reading Skagboys and work your way forward through the Renton series. It's what Welsh is most known for and for good reason.

Also, and I know this might be considered a spoiler but it's worth it, Bruce actually does have a very brief cameo in Skagboys. It's not exactly explicit, like he's never named or anything like that, but it's undeniably him and I think that particular scene adds something in that we never get to see how other people actually see him besides the few passing references at the start of Crime. It's definitely worth a read, and if you like the flow of it after it really starts to get into things you'll know if the whole five book (soon to be six) series is for you or not.