Beautifully explained with a proper amount of detail. It's up there with Horowitz and Hill's Art of Electronics in terms of clarity, but I would suggest makes for an even more thorough introduction to electronics. The description and explanation of transistors (both BJTs and FETs) and their behaviour is the best that I have read - it's exemplary! I would also say (as an audio engineer) that it actually equips you better for the design process than even Horowitz and Hill, despite their emphasis on the practical, which is already very good. There seems to me to be a slightly fuller explanation for the reasons that inform your choices in design, before going to an example circuit. H&H are very good at giving you practical circuits, and quite a breadth of them, which is immensely useful, but you won't necessarily grasp why all the component values are as they are. Nor does the additional detail here make understanding any harder at all; in fact it cements what you have learned. This is a model textbook (and, believe me, they are not all this good - in fact some are truly awful, including from academics) and, from the audio perspective, you will get more from this book than from specialist audio authors - excepting Bob Cordell's Power Amplifier book, which does a somewhat different thing anyway, but is one of the few not replete with mistakes.
The book is suitable from perhaps 'O' level upwards, though there is no reason a suitably bright 14 year-old shouldn't manage it. I certainly haven't seen anything that an 'A' Level student couldn't manage and, in both cases, it is probably the quantity of information, rather than its difficulty, that will be the limiting factor. In fact, if you master what's in this book, you will probably have a better understanding of the field than most graduates of related subjects these days. (Yes, that is an accurate reflection of how far the standards have fallen.) I certainly wish that I had come across it back when it was published in 1977; it would have filled in the gaps left by Nuffield Physics 'A' Level (the first debasement of science teaching) and I wouldn't have noticed the chasm between that and the standard demanded by Oxbridge Entrance then.
I cannot recommend this book enough. It is a must for students and for young professionals entering the field - it should be in every company library - along with H&H. What I find to be an utter tragedy is that the copy I bought is marked "Withdrawn", from The University Library, Leeds. I can hope that they bought later editions, but I somehow suspect that's probably not the case.