Welcome Coffecat. The four main books on the case are:
The Murders At White House Farm-Carol Ann Lee. A forensic approach, very well documented and I could recommend.
In Search of the Rainbow's End-Colin Caffell. Heartfelt, moving. One man's effort to find something positive from the wreckage.
Blood Relations-Roger Wilkes. A chronological account of how the murders came about. Good forensically discussing the aftermath of the trial and its implications.
Murder at White House Farm-Clare Powell. A more personal, intimate account, especially good on Sheila.
There's a good thread started by egap1, one of the best threads here, entitled Murder/Suicide at White House Farm, but I can't locate it.