As noted in the previous piece about Gallows Court I did indeed get the second book, Mortmain Hall. Martin Edwards, who is the acknowledged expert in all things Golden Age mysteries and the crime novel genre in general, sets up a fascinating story around murderers who have apparently gotten away with their crimes.
His knowledge of England, especially London, in the first third of the twentieth century really gives this story its color and feel. I rather enjoyed being brought right into 1930 and following Jacob Flint, Rachel Savernake, and the Truemans around as they untangle this particularly complicated plot. He nicely brings in the characters introduced in Gallows Court so that it seems as though you just turned the next page from the end of that novel. So yes, you should read Gallows Court before you begin Mortmain Hall.
I won’t say more about Mortmain Hall here, as even the tiniest revelation could spoil the whole adventure! But I did find this one a bit more satisfactory in terms of following all the clues to the resolution. At the end, Edwards adds information from his research about the Golden Age by inserting “clue finders” into the novel, and then listing them and explaining about it in an appendix. A really fun thing to learn!