Answer: There is a web site called the Time/Life Plant Encyclopedia that has lots of useful information, including basic soil preferences. You can search by plant name to find that plant's particular needs: http://pathfinder.com/cgi-bin/VG/vg?03
I don't know of any book that specifically targets soil preferences; most give information about climate preferences, cultural practices, hardiness, pest problems, etc. and list soil needs as well. Soil science isn't that specific and "scientific"--by that I mean that you won't find a reference that says daylilies like a pH of 6.4, while asters like 6.2). Preferred pH is usually given as a range, and usually hovers right around neutral (7) or slightly acidic. Only when plants show a marked preference for acidic or alkaline soil is pH emphasized (such as azaleas, blueberries.)
There are some generalizations you can make about plants' preferred soil types--some plants prefer a slighly alkaline, relatively dry soil, while others need a somewhat acidic soil that stays consistently moist for example. But any good reference guide will give you this information. Since you don't specify just what plants you're researching, I can recommend a good, all-around garden book called America's Garden Book, from the Brooklyn Botanic Garden. It's quite a large book (and expensive) but it lists thousand and thousands of plants and their general requirements (as well as chapters on garden design and cultural practices.) I hope this helps!
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