I prefer the first Glittering Images book because it focuses on the main directors (and the most significant films), giving them a chapter each. The second one is a bit choppier, covering lots of lesser known films in no particular order. I passed on buying the third one, as the law of diminishing returns appeared to be kicking in by that point. The books are lavishly illustrated and have text in Italian and English, though the English has clearly been written by Italians and it makes you grit your teeth from time to time (I'm sure one of us would have subbed it for free, if only they'd asked!)
For me, the groundbreaking SW book was Staig & Williams's Italian Western: The Opera of Violence, (1975) which opened up the whole world of the genre for me. The text is a bit chaotic, but it's packed with rare stills, has detailed UK release info, and there is no better source of background info on SW music and composers.