As others have pointed out, advice like "Be happy!" and "Stop self pity!" are all very well, but without practical advice or suggestions as to how to get out of those ruts, this book is all but useless.
Anyone who's in a really bad place or suffering from depression, when told "Be happy!" is likely to just feel guilty because it's blimmin' impossible to "snap out of it", and this whole book could be summarised as "Snap out of it, stop worrying, be happy!"
If you're a basically happy person who wants some rather shallow little phrases to pep you up, this is likely just the ticket. Otherwise, you can find more information and more actual help for free just by googling "self help tips".