These are the ones I went for but remember I am in the Uk so I have included which plants each book focuses on:
Harry Tomlinson, Pocket Encyclopedia of Bonsai, Dorling Kindersley Publishers - excellent for taking with you to nurseries or plant centres to see if material is suitable. Comprehensive list of plant species and very nice picture sequences to show you how to achieve certain styles. Includes plants from all over the world.
Peter D. Adams, Bonsai Design, Ward Lock Publishers More advanced book focussing on how to design and improve bonsai. Gives 5 year developmental plans for Beech, Hornbeam, Zelkova, Juniper and Cryptomeria. This is for when you are ready to grow bonsai in the ground (for bulk) then refine it's image over several years.
Peter D. Adams, The Art of Flowering Bonsai, Ward Lock Publishers. Excellent book for getting the most out of flowering Bonsai, features these plants:
Azaleas (Kurume and Satsuki)
Susan M. Bachenheimer Resnick, Bonsai, Tiger Books International.
Some outstanding specimens and sections on advanced techniques like Root over Rock. Based in New York many of the specimens are taken from the outstanding collection of bonsai in The Brooklyn Botanic Gardens.
Good luck hun