I definitely recommend this book. It will make you a better navigator even if you don't adopt the low-tech navigation solutions he recommends because it will clarify the origins/basis of major navigational concepts, such as latitude and longitude, etc.
One thing worthy of note. There are approximately four places where the (nicely printed) text flatly contradicts itself. A diagram will incorrectly refer to the "Southwesterly" trade winds immediately after the text clarifies that they are "Southeasterly." This can raise confusion about the point that section tries to make, which is that winds are named from the direction they come FROM, not the direction they blow TOWARD. The diagram will suggest the opposite (wrong) answer. Elsewhere, there are several contradictions about Zenith Distance being positive or negative. The picture or text will say that ZD is positive, immediately followed by a caption that tells you it is negative, or vice versa. These are small blemishes on an otherwise brilliant and fine book, but the reader should be aware of them.
Some of the other moments are tricky to follow, but that is due to the complexity of the subject. Lagan is a good, clear writer. If you read him patiently (and go over difficult concepts a few times), all the information you need is there. This is a truly fascinating look at pre-technology (and for much of it, pre-compass) navigation. Well worth having on your shelf.