Hi Kasha,
That's a great move you're making, whether it's for your own health, or for the environment, or for the animals!
These days it's super easy to be vegetarian because there are so many substitutes. You'll find as you continue cooking veggie you won't need them, but they are great to transition, and they are also great convenient food. Instead of putting meat in your cart, put in Gardein brand chickn, Field Roast sausages, Boca Ground Crumbles or burgers. There are just so many options. (They all taste great, but enjoy them for their own flavors, not expecting them to be the same as meat.) Or you can use seitan or tofu depending on the recipe.
Same for veggie cheese and non-dairy milks.
I would suggest that you start by using them as ingredients instead of sitting as the principle item on your plate. Basically that opens up every casserole in the world to you.
Lasagnas and pasta are easy to make veggie, same with Mexican food and really just about any 'ethnic' cuisine. Quiches, breakfasts, salads and sides are easy. Actually now that I think about it, there's probably a vegetarian version of just about any recipe you can think of. And don't be afraid to look at vegan recipes. Some people have a fear that vegan = bland, but that's so far from the truth. Eggs and milk are the easiest things to replace, so nowdays, vegetarian and vegan recipes are just about interchangeable.
As far as cookbooks, that's hard to say not knowing if you like to cook or "have" to cook. I really like Veganomicon, Hearty Meals for Monster Appetites, 30-minute Vegan/Vegetarian, Vegan Cookies Invade Your Cookies Jar, Vegan Table.
And there a ton of good websites and blogs -- vegetariantimes.com, vegnews.com, ohsheglows.com, lunchboxbunch.com, findingvegan.com, and lots more.
One of my never fail recipes is
Mini Vegan Wellingtons. I also just fell in love with
Vegetarian Stuffed Cabbage (Ww). There are many more, but I'll try to hold back since you really were asking about cookbooks.
I usually join in on the monthy recipe swap in this forum because it gives me lots of new ideas. I see the sign up thread is posted so why not jump in. It's super simple -- not one of those huge games with complicated rules. Just pick a listed chef and make two recipes.
Hope that is a helpful start, if you have any questions, just post them, and I'm sure you'll find a variety of options to get you thinking.