It hurts me to see negative Amazon reviews because this is one of the single best references on market risk VaR (i.e., market VaR not credit VaR). I do agree with previous reviewers is three respects: Dowd's book has a specific non-beginner audience, this is a theoretical (academic) rather than practice-oriented text on VaR, and its strength is not really portfolio VaR. Okay, so clearly this is not really for an introduction to VaR nor is it for VaR-in-the-trenches.
But within the (advertised) scope, there simply isn't a better survey of theoretical market risk VaR tools. Dowd goes into much more detail that Jorion (who is a leading author). And, very few will appreciate the analytical rigor and precision of Dowd; I use Dowd to teach VaR, and where other authors make formula mistakes, he doesn't seem to. Other authors, like Culp and even Wilmott, use a VaR that leads new learners into certain math errors, but Dowd's VaR formulations are thoughtful and less prone to deployment errors. Each year I use this book, it continues to impress me more.
Special strenghts are: reviews of non-normal parametric value at risk; succinct and accessible introduction to extreme value theory (EVT); careful review of VaR's weaknesses (e.g., non coherent)--Dowd is expert on the limitations of VaR; and liquidity-adjusted VaR.
The bent is mathematical, for sure, but most of the book is within reach of basic/intermediate calculus; if you have a background, you'll find that some of the first one-third is even probably too much introduction (e.g., volatility, covariances and correlations).
The FRM (financial risk manager) has assigned only Chapter 16 (Model Risk) to FRM candidates. Which is ironic, because this chapter isn't so strong, FRM candidates should be reading the rest of the book. But, for now, three years on, this is still the leading book in a very specific category.