For CSS one has to read a lot of books. Not a single book cover whole syllabus of each subject after change of syllabus in 2016. If such books exists they are mostly from local authors completely copy pasted from net. So, purchasing huge number of books is a great problem and big burden on pocket. It is also a reality that without hard copies it is difficult to prepare important examination like CSS so one should buy at least one book in hard form of each subject.
Figure out which books you need more for preparing your selected subjects and buy accordingly.
Last year I had purchased 95% of my books from following online website. It is one of the largest book stores located in Karachi. The only problem is that they offer cash on delivery only in Karachi and people of other cities have to transfer amount first. After confirmation they send you books with a week. For me book store was reliable. I had placed order of huge amount and my order was received on time and just the way I wanted.
The main advantage of the website was that they have almost all the books of top publishers in Pakistan like JWT, Carvan etc. They also have highly recommended books by foreign authors. You can get most of the books you desire from this website.

http://www.cbpbook.com/

Book list is highly organized. You can easily find them. Also monthly magazines like JWT monthly magazines are also available on this website.

If you want to purchase by self visiting then Lahore city is hub for that. You can visit Urdu Bazar. Almost all books can be easily purchased from there.

Hope I have sufficiently answered your query.

Please do share soft forms of books which you have for the benefit of others.