Ten years ago I took a series of mathematics courses with the Open University (a distance-learning university based in the UK). I had always wanted to take my understanding of mathematics beyond what I had done at A-Level, and this seemed to be the best way to do it. I did much of the course work on the trains to and from my work. This was back in the bad old days when British trains were subject to frequent delays and cancellations. However, when my train was late I just used the time to do work that I would have otherwise had to do in the evening or at the weekend, so in a way, I turned the delays to my advantage.
One of my aims in taking these courses was to get a basic understanding of group theory, which I kept coming across in my reading. The Open University courses did cover group theory, but we ran through it rather quickly so that, although could answer questions on various parts of the subject, I didn't have any overall feel for how the parts fitted together. I wasn't until I read Unknown Quantity by John Derbyshire and Finding Moonshine by Marcus du Sautoy that my course notes started make sense to me.
Both books are rather similar in the area they cover and in the level at which they cover it: Unknown Quantity is a history of algebra in general, Finding Moonshine is a history of the more restricted field of group theory. Du Sautoy has the advantage over Derbyshire in that he is an actual practising group theorist (at least until recently) and he supplies some interesting insider details on the Cambridge University group theorists and John Horton Conway in particular. However, this advantage is counterbalanced a little by Du Sautoy's tendency to get diverted into the arty side of symmetry. Still, both books are excellent and I would heartily recommend both to anyone studying or intending to study algebra at university undergraduate level.