The Sorcerer's Companion: A Guide to the Magical World of Harry Potter has been on the New York Times bestseller list and is being published in numerous foreign language editions, including Spanish, French, German, Italian, Dutch, Japanese, Polish, Portuguese, Czech, and Croatian. Here's what the critics have to say:
 

From School Library Journal: 
Gr 4 Up-Anyone who thinks that grims, grindylows, hinkypunks, and hippogriffs were invented by J. K. Rowling will be surprised by this offering.  Each of the 84 alphabetical entries describes a subject studied (arithmancy, divination), an object used (wand, cauldron), or a creature that appears in at least one of the Potter books, including a specific reference to the title, chapter, and page number. Then, using references from ancient, medieval, and modern literature, legend, mythology, and religion (almost 100 sources are listed in the extensive bibliography), each entry goes on to describe where, when, and how that subject, object, or creature has also been part of Western history, literature, or oral tradition.  Most entries are from one to three pages long, with some notable exceptions-12 pages on magic and 14 on magicians throughout history.  A fascinating history of the uses and abuses of medieval alchemy is buried within the article on the sorcerer's stone, but most subjects are easy to find.  Sidebars highlight specific aspects in detail-the mummy's curse, alchemical frauds, animals on trial.  More than half of the entries are illustrated with black-and-white drawings, woodcuts, or reproductions.  The material is interesting and informative, easy to read, and fairly wide-ranging.  The same readers who can't get enough of Harry Potter will enjoy learning about arithmancy, spell casting, and much more here, while waiting anxiously for the next Rowling novel to appear.  - Susan L. Rogers

From Publishers Weekly: 
Father/daughter team Allan Zola Kronzek and Elizabeth Kronzek target the wizard's mature fans with The Sorcerer's Companion: A Guide to the Magical World of Harry Potter. Each alphabetically organized entry contains a potent blend of fact, fiction and folklore.  The "Broomstick" entry, for instance, details the effects of a purported "flying ointment" that witches rub into their brooms to prepare for takeoff.  The section on "The Sorcerer's Stone" explains the ancient Egyptian art of alchemy.  A note at the end of each section shows readers where to find the reference in the Harry Potter books.  Thorough research and period prints combine to create a memorable book. 

From VOYA (Voice of Youth Advocates, a library journal): 
This literary guide explores the history behind all of the fantastic and mystical elements mentioned in the four Harry Potter novels.  Alphabetical and encyclopedic in its design, it should be browsed randomly, rather than read straight through.  Typical entries include objects (talismans and charms), lots of magic (hexes, curses, spells, and potions), crafts (astrology, rune casting, palmistry), and a full zoo of creatures (dragons, giants, grindylows, basilisks, and veelas).  These features only scratch the surface of what is covered in the book.  All subjects are briefly compared and contrasted as the history of each is traced.  Helpful to Potter-philes is a code at the end of each entry that directs the reader to the particular Rowling novel and exact page number wherein the subject is mentioned.  The elder Kronzek in this father/daughter team is a frequent lecturer on the elementary school circuit, and together they produce an attractive text, easily accessible to the young reader.  The layout of the book is enhanced with illustrations and woodcuts, many dating back to sixteenth-century works.  Its bibliography is equally impressive, citing more than one hundred sources from historical to Web based.  The guide is an entertaining overview of European lore and mythology.  One cannot help but appreciate the extensive research that J. K. Rowling delved into to incorporate such a variety of creatures and crafts into her work.  Some parents, however, might have reservations about the easy manner in which this book instructs and perhaps legitimizes some of the crafts it explores, such as reading tea leaves, doing numerology, and casting spells.  Nevertheless this book is a match for circulating collections in which the Potter books are popular.  - Kevin Beach, February 2002 (Vol. 24, No.6)

From Amazon.com: 
Harry Potter aficionados: remember when Buckbeak, Hagrid's pet Hippogriff, was put on trial by the Committee for the Disposal of Dangerous Creatures?  This crazy idea was not invented by Harry Potter's creator, J.K. Rowling.  In fact, from medieval times all the way up to the 19th century, animals and even insects were often charged with crimes, arrested, imprisoned, tried, convicted, and sometimes executed.  Harry Potter's fantastic world of magic has its roots in true history, mythology, and folklore; father-daughter team Allan Zola Kronzek and Elizabeth Kronzek have now made this wealth of astonishing information available to Muggles in their Sorcerer's Companion.  From astrology to Grindylow to reading tea leaves to witch persecution, this fascinating volume gets to the bottom of every magical mystery connected with Hogwarts.  Readers learn the unusual method by which premodern Europeans protected themselves from the cry of the uprooted Mandrake, involving a loyal dog and a rope.  (Professor Sprout's solution was to have her herbology students wear earmuffs).  Hermione probably knew, when she was hexed by Draco Malfoy so that her teeth suddenly grew past her chin, that hexes originated in Europe.  But did she know the connection between hexes and the folk magic of the Pennsylvania Dutch?  For fans of the tremendously popular Harry Potter series, or anyone who is intrigued by magical lore, the Sorcerer's Companion will quickly become a true friend.  (Ages 9 and older) --Emilie Coulter 

From Parentís Guide to Childrenís Media, Inc:
Before heading to Platform 9 3/4, new students at the Hogwarts School of Witchcraft and Wizardry will want to pick up a copy of The Sorcerer's Companion. Readers will discover the history of witches and wizards, potions and charms, magical creatures and crystal balls. The Kronzeks have created a quick and easy reference guide to the history, legend and literature included in the Harry Potter series. - Deborah West 

From BookPage: 
Add to your Hogwarts reference library with The Sorcerer's Companion: A Guide to the Magical World of Harry Potter by Allan Zola Kronzek and Elizabeth Kronzek, a father and daughter team who share a love of magic, history and Harry Potter.  Topics in the book range from arithmancy (Hermioneís favorite form of divination) to zombies, as the authors reveal the origins of potions, spells, hippogriffs, wands, boggarts, broomsticks and much more.  In The Sorcerer's Companion, discover the true history of Nicholas Flamel and find out how the Sorcerer's Stone got its power.  A note at the end of each section shows readers where to find the reference in the Harry Potter books.  Even Professor Quirrell would be impressed.