The typical 19th/early 20th Century memoir is always suspect. The tendency or trend at the time was to exaggerate one's accomplishments - often (see Sarah Bernhardt) to the extent of patent fantasy - for effect. I can't, at this late date and without extensive research, say whether or not Siringo's memoir fits neatly into this niche or not, but to his credit, it reads as a realistic, down-to-earth record of actual events, if mostly those which would do his reputation credit.
One reads a work of this nature for Adventure! Thrills! Guns!, and this certainly delivers. But it's much more: a travelogue; a character study (as we meet the various folk Siringo encounters and befriends on both sides of the law); and, indeed, a fast-paced journey through a variety of different "cases". The author proves to be a gifted storyteller, whose pacing, often genius-level use of segues, and humorous inclusion of Cowboy expressions make for an enjoyable read.
Siringo seems not to have outspokenly demeaned others on basis of race or origin, but there are stereotypical descriptions and language which may be offensive. Hunting is described within the text, though, with the exception of one buffalo hunt, it's fairly matter-of-fact and not graphic. There is also kindness, as Siringo portrays himself as an animal lover and had many pets to whom he fondly refers throughout the text.
The Pinkerton organization was understandably disturbed at Siringo's recounting of cases he had worked during his career with the agency, and in fact blocked publication, until all references to "the Pinkertons" were replaced with a euphemism ("the Dickensons"). At this late date, the distinction may seem humorous, but to the fan of history: is this confirmation that the stories are all true, or a clever marketing ploy intended to stimulate interest in the book?
I'd like to also add: the version I read is a Google rendered edition. This means that it was generated through a combination of OCR (Optical Recognition Rendering) and ReCaptcha, which further means that there are some wacky seemingly-typographical errors. For myself, knowing that this was the process (and that I, as a consequence, was able to read this delightful work for less than a buffalo nickel), it wasn't a distraction.
In short, if this sounds like something you would like, I think you will like it. You can get it for free; give it a try.