WGPO members and ardent word enthusiasts Maliha Mahmood and Norman Wei have recently published books that are entertaining study guides. Both are available at Amazon.com.
The Plant Kingdom Word List is an alphabetical listing of plants and plant-associated words, accompanied by lovely illustrations.
Word Games Anagrams and More enhances anagram study with humorous illustrations, and has a number of other tricks for learning and retaining unusual words. See details at www.normanwei.com.