Karl Baedeker (
not Baedecker) (1801-1859) was the publisher of authoritative guidebooks for tourists. They contain important introductions, descriptions of buildings, of museum collections, etc., written by the best specialists, and they are frequently revised in order to be up to date. For the convenience of travellers, they are in a handy format and in small print.
Such a book is often referred to as "a Baedeker", and sometimes the term is used about similar works from other publishers.