H.P. Lovecraft in Britain (2007), newly republished in summer 2020 as a budget Kindle ebook for £2.28. It’s paired with “The Horror in the Museum” for some reason, though it appears the latter is not annotated or also given an essay.
The long essay untangled Lovecraft’s British publishing history from the 1950s to the 70s, with the help of the Gollancz archives. It was originally published as a limited edition 46-page chapbook from The British Fantasy Society, with a pleasing cover and interior illustrations.

It looks like it may still be possible to get a copy of the original chapbook monograph from the author, for a mere £6 inc. UK postage. As such it may be rather more desirable than the new ebook for many Lovecraftians. Worth a try.
Also new, in a 300-page paperback, is Claudio Foti’s book in Italian on Lovecraft and Kenneth Sterling. This was published at the end of June 2020, and has an essay on the pair, “together with all of Lovecraft’s letters to Kenneth J. Sterling” in translation. It looks like it may also have a translation of “Eryx”, and hints at possibly containing other Sterling story translations in Italian. Foci has been a Lovecraft Annual contributor (2017) and hopefully we’ll get an English translation of his essay in due course.