Some of the highly regarded Donald Westlake Parker novels include 'The Hunter'. It's a great example of his work as it features Parker's unwavering and calculating nature in his pursuit of revenge. Parker is a complex anti - hero, and this novel really sets the tone for the series.
Well, the number of Parker novels written by Westlake varies depending on different sources. Some say around a dozen, but it's best to refer to authoritative literary records for an accurate count.
I'd recommend starting with 'The Hunter'. It's the first one in the series and gives you a great introduction to Parker's character. You'll quickly understand his motives and his no - nonsense approach to getting what he wants.