The following are a list of books featuring Sonic the Hedgehog:
Stay Sonic by Mike Pattenden was published in the UK in 1993 by Penguin Group under their "Fantail" imprint, which is itself part of their "Puffin" imprint. Stay Sonic is a handbook featuring numerous sections, including a retelling of the "Kintobor origin" for Sonic and Dr. Robotnik that had originally been established in Sega of America's "Sonic Bible" documents and had subsequently been featured in the Sonic promotional comic. Other sections of the book include guides to the zones of Sonic 1 and Sonic 2, and guides to the boss fights in the 8- and 16-bit versions of Sonic 2.
James Wallis, Marc Gascoigne and Carl Sargent (under the pseudonym of "Martin Adams") wrote four Sonic the Hedgehog novels based on the origin established in Stay Sonic. They were published by Virgin Publishing.
Michael Teitelbaum also worked on the following related books under Troll Associates.
A series of 3 books written by John Michlig and illustrated by Art Mawhinney were published by Golden Books Entertainment under their "Look Look Books" banner. Originally available through Scholastic book clubs, these books have sold a combined total of over half a million copies worldwide. While the two who worked on the books never met, Michlig later praised the way Mawhinney drew out his scripts. 
A number of gamebooks were published in the mid-90's for the U.K. market featuring Sonic the Hedgehog.
Ladybird published a series of four "adventure gamebooks", which are dissimilar to the typical format in that they're largely pictorial and involve instructions such as "Count up the flying badniks – the total is the number of the next page."
Once again under their "Puffin" imprint, Penguin published a series of six Sonic Adventures gamebooks (unrelated to the similarly-titled game) in the traditional format.
Once again, Ladybird used the Sonic the Hedgehog license to release a series of "Puzzle Books," which were a series of activities and riddles.
Ladybird also published a series of "find the hidden items" books in the style of the Where's Wally?/Where's Waldo? franchise
This is a list of other assorted Sonic the Hedgehog books that don't fit into the above categories.