One of the best books I've read on the subject. I'm an on again, off again vegetarian. Did it for a five year stint once. You might need to supplement with B12 and D (the latter if you don't spend much time outdoors). You won't lose bone density or muscle mass. You don't weaken your immune system. You don't mess with your hormones. It has zero impact on hair loss (WTFLOL). That is some 1960s level meat industry propaganda bullshit if I've ever seen it.
I'm a research guy. When I commit to something, it's only after hours upon hours of reading up on peer reviewed studies and the like. There's conflicting data about almost everything, but the facts show that vegetarians and pescatarians live on average longer than more traditional omnivores. There are obviously gradients in between, and a vegetarian can eat a pure garbage diet and get diabetes and heart disease just as likely as a meat eater. The largest study on the topic shows 20% difference in mortality rates between the two groups. There are hundreds of other studies showing the same, even after controlling for other environmental factors. Vegetarian diets are very, very healthy. You just have to mix and max different sources of protein so you get full amino acid coverage.
http://ajcn.nutrition.org/content/100/Supplement_1/490S
I never did it for health, it was purely environmental concerns. Water waste and greenhouse gas. Unfortunately, I'm allergic to celiac (not on the gluten free bandwagon, I legitimately have an allergy and it sucks) and have some sensitivities to dairy products, so a lot of the meat substitute stuff doesn't work for me. Now I'm good with eating the occasional antibiotic-free chicken breast. Yum, chicken. I avoid red meat almost entirely, but I might cheat once a month or so. Meat isn't inherently bad for you. Vegetarian diets are certainly not bad for you. Moderation and common sense are the key.