For this particular question, I don't think the requirements are specific enough to warrant reopening it -- "hiking" and "castles" seem to be the only real requirements. Pretty sure that any extensive writeup on the topic should suit the OP. And most of those books would be illustrated anyway.
A cursory search on Amazon for "Germany castles" yielded sufficient number of results. Unless the OP is really into castles (and if they are, this is not the most suitable place to ask), I'm sure they would be able to find an adequate book on the topic -- there are enough comments, recommendations and reviews on most of the titles, from people who actually bought and read the books.
Generally, finding a good book on a particular topic is your classic shopping recommendation question -- it's very easy to find loads of them, and what is "good" is very subjective. Unless very specific criteria are listed and the author demonstrates that they did searched for a book covering their favourite topic, these questions are more of an excuse not to do one's research and should be closed.