Well, depends on the scope of what you're looking for. Tucson Cactus and Succulent Society put out a pretty comprehensive field guide but it's limited to cacti and succulents native to Arizona. Cactiguide has images and taxonomic notes, but it doesn't really have descriptions on how to care for the plants. This book is pretty comprehensive when it comes to Agaves. There are other books specific for different genera, which is why you'll find us plant enthusiasts have multiple books.