Often times the key to finding these secondary sources is finding the right keyword. In the case of online research Google Scholar is your very good friend. It not only vets scholarly publications, it's also a quick and dirty way to vet how influential a particular work is. Note, for brevity, I'm just going to look at the first page- after all I'm not going to do your research for you! You should always look at multiple pages and play around with the publication date.

In the case of your topic. I ran the search "court architecture" and came up with several results after "early modern palaces" didn't give me much. Sometimes you'll get lucky. For example, one of the first hits in the previous search is The Royal Palaces if Tudor Architecture which is cited by 214 scholars. Other times, the right keyword leads to more keywords. Right below the Tudor book is Architecture, Ceremonial, and Power: The Topkapi Palace in the Fifteenth and Sixteenth Centuries and it made me modify my search into "court architecture ceremonial". You'll note that some of the top hits don't quite fit in with your topic, so you might need to dig around. Towards the bottom of the page, you'll see The utility of splendor: ceremony, social life, and architecture at the court of Bavaria, 1600-1800. Even though it is not as widely cited as the two other books, that is not a sign that it is automatically a bad book. Some titles are highly specialized and don't penetrate the wider consciousness of the academic community beyond those narrow specialist circles.

The next step when you get a booklist is to search reviews of them on jstor. It actually might help to have jstor's advanced search up in a separate tab. So you see a book that peaks your interest, copy&paste the title and limit your search to reviews. The reviews might tell you if the scholarly response was tepid, enthusiastic, or negative. Remember, don't just read one review or judge a book based on one review.

Once you have the books in hand, search two things. Firstly, the bibliography/works cited (unfortunately current academic publishers are starting to jettison the bibliography). These give you some idea of the wider scholarship. Second, parse out the acknowledgements, especially if it is the first or second book by the author. In between thanking foundations for giving them money and pleas of forgiveness of family members for putting up with the life of an academic, you'll find a listing of important scholars who've helped shepherd the research and publication process along. Some of these names might be valuable to track down and further help your research. The larger point is you want to keep digging as sources lead to more source.