Supernatural: Bobby Singer's Guide to Hunting (Paperback) by Reed David
Bobby Singer has been a hunter since Sam and Dean Winchester were in diapers and he's hunted every monster spirit and demon there is. He's become a surrogate father to Dean and Sam and their most trusted ally. They count on his knowledge to get them out of jams but now that knowledge is disappearing-something is stealing Bobby's memories
With only hours until he becomes a gibbering vegetable Bobby must write down everything he's learned in over twenty years on the job. If he fails all his experience will be gone forever. If he's lucky he may even track down the creature that got him in this mess in the first place.