Hour of Need is the concluding book in Michael Pryor's Laws of Magic's series and it brings a good series to a fitting and satisfying close. I won't bother recapping the general premise as you'll want to have read the prior (or should that be Pryor) books first so you can check out the setting etc. in my earlier reviews. Suffice to say it's set in an alternate Edwardian time period that has been moving toward and then finally arriving at their version of World War I involving both technology and magic.
All the major characters return for Hour of Need: Aubrey--whom I've described as a mix of Holmes, Tow Swift, and Hermione Granger; his best friend George--reliably strong and warm-hearted; Caroline--his crack shot, martial artist maybe-finally-girlfriend-this-book; and Sophie, coming into her own magically in this book as well as moving more seriously into her relationship with George. As one might expect in the conclusion, the villain--Dr. Tremaine--is here as well, and yes, he and Aubrey do have a final confrontation.
The book is darker that the earlier ones in the sense that while the first few were heading into the war and the last one has the group fighting at the fringes, here we see the actual front and it has all the horror as the non-alternate-universe WWI--the trenches, wire fencing, shell-shocked soldiers, are all here and it makes for a nice raising of the stakes as well as some strong, affecting imagery.
The grim nature and darker imagery in the middle section strengthen the book, as does the continued development of the relationship among the characters and the movement toward an awareness of the horror of war no matter which side (that isn't to say Pryor offers up any moral relativism here; he simply doesn't shy away from the sometimes monstrous things being done).
Hour of Need `s flaws will be familiar to readers of the series--though they are presented as difficult, it still feels that things come a bit too easily or quickly to Aubrey and that's true as well for Sophia, whose magic skills are moving perhaps a bit too fast. Though Pryor still does an excellent job in the presentation of the magic itself as a kind of science in its intellectual rigor. Tremaine remains a bit too abstract, the confrontation is a bit perfunctory, and this book, like others, could have lost 50-75 pages or so.
But overall, as mentioned, it's a fitting and satisfactory conclusion. I've called the series "charmingly old-fashioned" and it remains so to the end. Too old-fashioned for some I've no doubt, but personally I found it to be one of the series' greatest strengths and, to be fully honest, a bit of a relief amidst all the "gritty" or dystopic YA fiction. Having read it start to finish, I can happily recommend it.