One of Osprey's excellent series of military histories focusing on sieges in the Peninsula campaign, I bought this in advance of a holiday to Spain in 2007 prompted by my wife's desire to return to Salamanca to study Spanish. As a reader of all of Bernard Cornwell's Sharpe novels and other Peninsula histories, I bought this to prepare for visits to Badajoz and Ciudad Rodrigo.
An economically written and lavishly illustrated book: it includes modern photographs, early aerial photos from 1914 and Jac Weller's from the 1960s, contemporary prints and the late Bill Younghusband's own illustrations. Like others in the series, it was readable and informative; although there is some overlap with Fletcher's other Osprey books "Fortresses of the Peninsula War 1808-1814" and "Salamanca 1812".
I did get to Ciudad Rodrigo, and everything Fletcher described in the book could indeed be found on the ground. Sadly I did not make it to Badajoz - just too far, and Fletcher's own description of the expansion of this city beyond its C19 fortifications made it sound like it was not worth the effort.
Fletcher includes excellent 3-D maps of the sieges, but if there is one thing that I think he might usefully add it would be modern maps - ideally in this case the Spanish 1:50,000 maps, which are much harder to find than their British Ordnance Survey "Landranger" equivalents - so that a battlefield "tourer" would find it much easier to locate any historic remnants on the ground.
If you want to visit, or you want an accurate pictorial impression of the terrain and features over which Napoleonic armies fought, these books are excellent. They do, however, occupy a somewhat infertile plain between original sources (which now abound) and proper military history, and the so much more readable military historical fiction of Mr Cornwell and his emulators.