Bike maintenance is like a language: you learn it by doing it, not from a book. Except that without the book you won't know what to do in the first place.
Being able to look after a bike is a valuable skill, especially when you see what some bike chains charge you for a service - fifty quid for really, really basic stuff you can do easily at home in an hour. (Anyone who's ever been rushed two hundred quid for a basic survey on a house, which tells you things you could have worked out for yourself like how many rooms it's got, will know the feeling.)
Bicycle Maintenance is a new guidebook which tells you all this stuff. Clearly laid out in a magazine style, it covers the literal nuts and bolts, from simple cleaning and maintenance, through cable and bearing replacement, up to truing wheels. (This is the imperfect subjunctive of bike maintenance: too tricky for normal people in practice, but it's reassuring to know the theory.) It's written mainly from the mountain bike perspective, but addresses road bikes too (with their different gear shifters, for instance).
Getting confidence fixing bikes does need experience. The section on fixing punctures is fine, for instance, but the only way of learning how to put a tyre back on with your fingers, without needing tyre levers, is to do it: it's a craft. Adjusting indexing and gear ratios – which the book also covers comprehensively – is presumably the same, though that's a bridge too far for real cyclists like me. A wobbly bridge at that.
This looks a very good reference to have on your shelf. Bicycle Maintenance, by Guy Andrews and published by Dennis, is available from Amazon for £8.99. You may well see it in shops like Tesco and WH Smith, and it's also on sale through magbooks.com.
And no, if you buy it through that link, I don't get any commission. And no, I don't want to hear from anyone telling me how to set up affiliate links and sponsored advertisements to plaster my blog with in order to earn untold riches.