The stories of Mr. James are above reproach; unique in English literature for being subtle and creepy at the same time and shaped by James' knowledge of medieval history, English history, and the occult. The question for the reader is whether they want to pony up $20 for the Penguin annotated editions of James' tales, as the James repertoire is in the public domain and you could read many of these stories for free on the internet and / or get a $6 copy of the "Collected Ghost Stories" from Wordsworth Press and get 30 out of 33 of the stories featured in the two combined Penguin volumes. So the question then is are Joshi's notes and intros worth about $14?
S.T. Joshi is an immensely gifted editor and critic. His studies of the "Weird Tale" are modern classics in the field, and the immense work he has put into his Lovecraft bio and his annotated Lovecraft volumes are a paradigm. There is no doubt that if Mr. Joshi put the full focus of his attention on working with James' material that he could have easily justified the purchase price for these books. Unfortunately, Joshi, for whatever reasons, just went through the motions here and produced a fairly pedestrian work of annotation and criticism to accompany the text.
Joshi's annotated Lovecraft or annotated Blackwood (also available from Penguin) are superb works of annotation - each story has copious notes explaining themes and background of the work at hand. In approaching James though, Joshi appears dutiful at best or even bored. Many tales here have less than half a dozen bland notes, and many of the notes are nothing more than scutwork, translations and nutshell bios of historical figure mentioned.
This would be fine if James needed no annotation. (But then why buy these books at all?) The true issue is that James' work would indeed benefit from some first grade notes. For instance "The Stalls of Barchester Cathedral" is a story involving an ambitious clergyman who schemes to murder his inconvenient predecessor who is holding him from a promotion. The career of the sinister cleric has many subtle details that point up James' low opinion of him based on his theology and clerical administration policies.
For a modern reader who is not fully aware of Episcopalian intra-denominational quarrels in the late 19th Century, knowing this info will add a new level of depth and interest to the story. Instead, Joshi tells us none of this and simply translates a few Latin phrases and fleshes out a few Biblical citations, something the ordinary reader with internet access could do on their own with a $6 copy of the stories. In comparison, Joshi's notes in his annotated Lovecraft for "Herbert West Reanimator" (one of the slightest and most pulpy of fictions in the HPL body of work) are far more detailed, engaging, and affectionate.
Now you might say "well who cares about Episcopalian church governance squabbles of the 19th Century?" and indeed the Barchester story works very well even if the reader knows none of the "extrinsic" detail. But yet the entire point of an annotated edition is to add maximum depth and detail to a story for those interested in pursuing such a level of analysis. To provide a minimalist annotation defeats the entire purpose of the endeavor, as the reader may be better served by dispensing with the slight commentary offered and simply reading the work in question cheaply or for free while doing their own cursory research as needed for historical figures, translations, etc.
The problem may be that Joshi is well-known for his postulate that a Christian perspective is incompatible with effective horror writing. Joshi is a rather strident atheist and feels that atheism and similar godless perspective make for the most creative and interesting horror. I see his point, but yet the existence of effective horror by pious men like Hawthorne and Montague Rhodes James acts as a counterpoint to Joshi's thesis. This is not to say that Joshi sets out to sabotage James with lame notes - rather it perhaps shows why Joshi viewed this particular exercise as a bore and a task rather than a pleasure.
I do not wish to psychoanalyze the editor too much; my theory above may be entirely wrong. However, the heart of the matter is that if we compare Joshi's notes and analyses with James to that on the stories of the pantheistic Blackwood, the existensialist Lovecraft, or the atheist Ligotti, we see that he has done a much better job than here. Maybe this is due to the fact that he finds these others more personally simpatico, or perhaps he simply finds detailed textual analysis of James to be uninteresting.
In any case, the ultimate answer to the question of whether a reader should purchase these two volumes of James' ghost stories is probably not. The much cheaper Wordsworth edition (though less aesthetically pleasing) will offer all the pleasures of the original text (or 90% of them anyway), the somewhat cheaper Oxford World Classics Edition though offering only 20 or so tales has a far better intro and notes by Michael Cox, and for the ultimate discount, most of these tales are in the public domain and can be tracked down and read for free over the internet. Joshi, though ordinarily adding enough value to an annotated edition to justify a higher price, has fallen down on the job here and given us a bare-bones minimal effort annotation effort.