According to fantlab.ru, a Russian website that is a sort of encyclopaedia of fiction, the work is intended as an "afterword" to The War of the Worlds. Here is their synopsys, and my translation:
Памфлет Л. Лагина «Майор Велл Эндью, его наблюдения, переживания, мысли, надежды и далеко идущие планы, записанные им в течение последних пятнадцати дней его жизни» как бы продолжает роман Г. Уэллса «Война миров». Офицер английских войск, убеждённый в непобедимости агрессоров-марсиан, чтобы не погибнуть, готов стать их прислужником. Своё предательство он пытается оправдать приспособленческой философией.
The pamphlet of L. Lagin Major Well Andyou, his observations, worries, thoughts, hopes, and far-stretching plans, recorded by him in the last 15 days of his life is a continuation of sorts to H. G. Wells' The War of the Worlds. An officer of English military, convinced in the invincibility of Martian aggressors, is ready to become their servant in order to stay alive. He tries to explain his treachery using an "adaptability" philosophy.
Or from his Russian Wikipedia page (translation also mine; his English page does not contain this):
Особняком стоит сильная и острая повесть «Майор Велл Эндъю» (1962), своеобразное дополнение к «Войне миров» Г. Уэллса, посвящённая проблеме коллаборационизма.
A separate place should be reserved for Major Well Andyou (1962), a sort of an addition to The War of the Worlds by H. G. Wells, devoted to the problem of collaborationism.
The website lists this work as a part of an informal "cycle" of works further exploring the ideas of H. G. Wells. The story is widely available in Russian online, e.g. here and here. Sadly, an English translation does not seem to exist.
I will try to read the piece and provide a synopsis here, but I can't promise I'll do it this week.