Reine said:A lot of times I check out used book sites to find a bargain. Even if you don't know the title of a specific book, just type in a keyword. In this case "adeniums". Hope this helps.
BTW I have always been satisfied buying used books from these sites. Read description of condition to help you decide if a particular book is acceptable to you. Compare different sites for price, condition, shipping, etc.
http://www.abebooks.com/
http://www.bookfinder.com/
http://www.alibris.com/