I have not read the books, but I have read reviews of the Killing Lincoln and Killing Kennedy books. The basic gist of the reviews were, this is just warmed over versions of the standard stories, probably mostly written by ghost writers. E.g. nothing controversial, or interesting, about them, just an attempt to sell some books by putting O'Reilly's name on the cover and marketing them towards his target demographic (middle-to-old aged, white, Christian American men). They sound dull, but at least devoid of revisionism or controversy. But, again, I haven't read the books personally. Just recycled bits from other popular accounts. Which is at least not terrible.