So what a pleasant shock to find that the latest Bridget Jones installment, “Mad About the Boy,” is not only sharp and humorous, despite its heroine’s aged circumstances, but also snappily written, observationally astute and at times genuinely moving. Fielding has somehow pulled off the neat trick of holding to her initial premise — single woman looks for romance — while allowing her heroine to grow up into someone funnier and more interesting than she was before. Who knew middle age could be so eventful?