As other reviewers have mentioned this story takes a while to get going. As much as I enjoy Liane Moriarty it was probably just a little bit too drawn out. There's suspense and there's just getting annoyed because you want to get to the point of the story. However when you get there Truly, Madly, Guilty delivers what it promises - a tangled story of three couples and the impact of a careless moment. As always Moriarty delivers well developed, three dimensional characters and tells the story in her trademark witty yet occasionally dark style. Perhaps Truly, Madly, Guilty's best trait is it's plausibility - anyone who reads the story will no doubt recognise how easily the incident at the BBQ could happen.
All in all this was an enjoyable read that kept me entertained from start to finish. As mentioned the lead up to the reveal of the incident could have been condensed a little but if you are a Liane Moriarty fan, you will be satisfied by her latest offering.
Warning: Remainder of reviewer contains a spoiler
One a separate note - one comment I would make is the CPR information that forms part of the storyline is out of date. CPR should be performed at the rate of 30:2 (compressions/breaths) rather than 15:2. I was a little surprised this was was not verified before publication as it has been this way for several years.