I used to, but in honesty the pay and progression wasn't great, but I did love it. I found that although there's a lot of variety in the job, it's also very repetitive. All that I have worked in used an hourly timetable so you knew what you were doing, the main jobs being:
Shelving: as it sounds, taking the trolley of returns and putting them in the correct places on the shelves. They should provide a trolley, and depending on the size of the library, if you sort the trolley prior to starting you don't have to be walking back and forth. However, you might need to do some bending and reaching. Everyone I worked with worked at a different pace, not because anyone was lazy, just obviously some people were as quick physically; this was never an issue, so I wouldn't let it put you off.
Manning the desk: as it sounds- answering enquiries, this could be anything from book recommendations to council and the local area questions. Obviously though if you don't know the answers, can either Google or ask someone else. You get to know the kind of enquiries you are likely to get, we were by a bus stop and used to have lots of people asking for the times as they couldn't read the tiny writing; so we had a drawer full of timetables printed in large print ready. This also involves joining new members and processing fines etc, you need to be able to use a computer but not to a particularly high level, you will be trained.
Assisting people on the computers: this is a huge part of library work now, and as they provide free computer and internet usage, you get asked a lot to help with applications and printing etc. It's good to know a bit about computers, but if you don't know a lot about certain processes, eg housing sites then there's normally leaflets or services to signpost people to. However, you will get asked the same questions over and over again, but it's lovely being able to support people, especially when they come back and let you know they've got the job.
Rhyme time: I enjoyed rhyme time, but no one else did so I ended up doing them all, which was fine, but it was daunting and nerve wracking to begin with. There are usually other activities and initiatives to get involved with as well, if you enjoy that kind of thing.
Misc: things like processing new stock and reservations, which is usually sitting down and you'll be trained in what to do. Also assisting with the self service machines.
Quite long, but I've worked for different councils and these seem to be common aspects of the job. A lot of staff are being replaced by volunteers, but there should still be the need for paid staff (hopefully), as volunteers can't use the computer system, for example, due to data protection (hence why volunteering is mainly just shelving).