I read The Gorse Trilogy after reading Hangover Square (my first Patrick Hamilton novel). I don't think these three books are as good as 'Hangover Square' however I think there is much to enjoy and they're far better than many would have you believe.
In common with 'Hangover Square', all of the books give the reader a wonderful insight into England in the 1920s and 1930s, in particular the highly delineated class structure. It is class-based assumptions that enable the books' anti-hero Ernest Ralph Gorse to successfully hoodwink those he encounters. As others have said the best book of the three is 'Unknown Assailant' (1955). Mrs Plumleigh-Bruce and her Reading-based companions are wonderful - and wonderfully funny. Gorse's early years are entertainingly evoked in 'The West Pier' (1951). As a Hove-based reader I particularly enjoyed this book. The final book, 'Unknown Assailant' (1955), is less successful but it still has its moments.
I wish Hamilton could have lived long enough to write more Gorse novels. He hints at what is to come but alas we will never know exactly how it all ended for our anti-hero. Despite this, I heartily recommend these books for anyone that has enjoyed other Hamilton works.