Yes, a letter exists, but to say it's notable or that it means that the highly speculative conclusion Cook makes is correct would be reckless and POV-pushing. On top of that, the claim that it was Best who wrote the Dear Boss letter based upon supposed similarities in handwriting is rather absurd considering that the supposed "handwriting expert" is not trained in comparing forensic handwriting analysis but graphology, a known pseudoscience... a simple look at the documents in question show that a number of letters do not match and the ones that seem to only do so within the same level of lots of other Victorian era writers. There's pretty much nothing new to Cook's research except for some very minor things that don't rate mention in an encyclopedia article.