This is a book on relativity "aimed at the pre-university age bracket, or at the general scientific reader who would like to have a fully correct treatment at an accessible mathematical level." Recent books intended for a similar audience are Takeuchi and Mermin. Steane's style is conversational and engaging. There is not as much of a feeling as in Mermin of being put through calisthenics by a stern geometry teacher. He nevertheless avoids the cutesiness of Takeuchi.
Steane got his degree at Oxford and teaches there. It shows. He assumes a highly intelligent reader with excellent preparation in math. The description of cosmic-ray muon showers, for example, makes the assumption that the reader knows the metric prefix micro-, understands about measuring probabilities using numbers from 0 to 1, and knows about multiplying probabilities of independent events.
The book is permeated by math, and I suspect that very few readers, even liberal arts students at Oxford, will be able to extract the meaning from the calculations as well as Steane apparently hopes. There are lots of fairly lengthy numerical and algebraic calculations, and the trouble is that Steane seems to assume that typical readers in his target audience are capable of going through such a calculation and abstracting from it the underlying ideas. This is not a skill that high schools anywhere in the world do a very good job of teaching. Most readers' eyes will just glaze over. If the reading is assigned for a course, then some will dutifully copy down the equations and memorize them, thinking that they might be asked to regurgitate them later or plug in numbers.
There are no end-of-chapter questions, although there are a few "puzzles" and "challenges" sprinkled around in the text. No answers are given, which would limit their usefulness to readers studying independently. They aren't numbered, so it would be awkward to try to assign them as exercises in a course.
Steane uses quite a few spacetime diagrams, but unlike Takeuchi and Mermin he doesn't use them very systematically, mostly only near the end of the book. This is unfortunate, because in my experience mathematically unsophisticated students often respond quite well to a systematic presentation in the graphical style.
The book is very disorganized. It skips around from topic to topic, and I can't discern any over-all blueprint. The lack of organization extends to smaller scales as well. Chapter 3 is an example. It starts with a section on cosmic-ray muon showers, but no conclusions are drawn. Then Steane has a section titled "The light from the stars," which says that we'd expect to be able to observe annual variations in the speed of starlight "using good optical instruments." Then he admits that this is not actually possible, but that we can do something similar using aberration of starlight. Then he passes to the Michelson-Morley experiment, which doesn't fall logically within the topic defined by the title of the section. Finally he has a section on Maxwell's equations, but describes them in a very vague way that no lay reader is likely to understand, and fails to connect them clearly to the preceding examples. The whole chapter is a muddle, with no clear points being made and no clear logical connections being drawn.
Steane makes a reasonable attempt to connect with experiment, which is more than Mermin or Takeuchi does. However, he isn't always successful. For example, his diagram of the Michelson-Morley experiment contains various elements that aren't explained, some of them extraneous (reflected rays that are not observed) and others not (a lens whose purpose the reader is unlikely to be able to infer, since it doesn't refract the rays that are drawn passing through it). There is a description of the Hafele-Keating experiment that I think few readers will understand, partly because it lacks a diagram.