Ruby Wax was the one who ran around shouting on TV a while back, extracting knicker-drawer secrets from celebrities. She was good at it too, even though the shows were always just as much about her as they were about Imelda's shoe habit or Pamela's top-dressing. But even then, Wax could do nothing to prevent episodes of depression so debilitating she often ended in the Priory. It was the voices that really did it, she says – not psychotic, just the ordinary nag-nag snip-snip internal monologue that most people think of as the sound of themselves.

Now, Wax has reinvented herself. True to high-achieving form, she's not content with becoming just a spokeswoman for mental illness, she's making a bid for full-scale poster-girl status. Which means her book is a curious mixture of old Ruby (frantic, arm-waving jokes, lots of me-me-me time) and new (Oxford degree, knowledgeable examination of the hippocampus, Ten Top Tips for Mindful Living).

Actually, a lot of this book is great. The first section – what depression looks like on a dark night – inevitably involves plenty of low puns and needy stuff. But the second, in which Wax gives the reader an account of the ways in which the brain controls and balances our emotional lives, is the clearest I've seen. Once she's calmed down and stopped doing the exclamation-mark dance, Wax is a generous, insightful guide. She's realistic, she doesn't talk down to her audience and she's genuinely enthusiastic about her new subject. The final part of the book is taken up with how to stop a galloping mind from bolting; useful if you've never heard of meditation, Buddhism or the countryside, not so great otherwise.

Somewhere among all the moments of really-don't-need-to-know honesty, there's a core of genuine hard-won sense here that could be of real use to anyone suffering from depression or mental illness. When Wax relaxes, she's great. But when she's "on", she's exhausting.