That second book sounds like it's been written for me! Will have to check out... I certainly have not found my niche, and always want to do everything rather than choose one thing.
I also really want to read the Happiness Project - keep coming across mentions of it and haven't got round to looking it out.
A charity shop find was perfect for me last year too - A woman in your own right by Anne Dickson. It's from the early '80s, so dated in its reference to how women are treated in society (but this was very interesting social history I thought) but spot on in its advice. I realised I am chronically unassertive, and just loved the clarity of her explanations of how communication should work and what assertiveness really is - it all just made perfect sense to me.
Also, Buddhism for Mothers, which I read recently and just felt like the right thing to read at that time - I can't remember quite what it was now that so appealed, but it was very comforting and refreshing and I could feel changes in my everyday life as I absorbed what it was saying.
I have two that I can think of The Happiness Project (apologies for the gratuitous link to my blog, but I am completely evangelical about this one!). I just finished this today and it was so inspiring. It's about making small changes in your life to make it better. The book is a memoir and the author approaches the project in a really methodical, goal-oriented way, which I love. I really identified with her, and really want to start my own Happiness Project.
The second for me is What to do when I want to do everything by Barbara Sher (also published as Refuse to Choose!). It's basically about people, whom she identfies as 'scanners' who try so many different things in life and end up feeling dissatisfied because they are not following a traditional path, they haven't found their 'niche' (hint: they may not actually have one - but that's ok - neither did Da Vinci or Michaelangelo). It was a book I found in a charity shop. One man's trash is really another woman's treasure. Except I think I lent it to someone who moved away...