This is my book collection. Some of my own and some borrowed from the library.
I started with simple ones. The first goal was to add more fiber to my diet. These two were good ones:

The 20/30 Fat & Fiber Diet Plan
High Fiber Cooking
The Fiber 35 Diet
The vegetarian books were the first ones to acquire. I still have some books adapting its recipes to vegan style. 
Classic Vegetarian Cookbook
Gardens Greens (all about salads)

A Passion for Vegetables
Fast Vegetarian (fast and simple vegetarian dishes)
The Joy of Eating Natural Foods
The Ultimate Vegetarian Cookbook (another book from Paul Gayler)
The Complete Encyclopedia of Vegetables and Vegetarian Cooking 
(the Bible of the Vegetarian cooking)
Complete Vegetarian Cookbook
I also like cooking dishes from other cultures. Although there are many dishes with meat, I modify them.
Good dishes from the Silk Road
Jose Andres (my preferred Spanish book)
El Farol (good tapas recipes)
As time went by and became vegan, I started with strict vegan books:
Chloe's Vegan Desserts
Artisan Vegan Cheese