hi kpcheah,
RF design and microwave design doesn't have a huge difference. If you want to do microstrip filter design, a book of lancaster is really a good reference. Also, "mathei" book can be a good but it can be more on advanced level. In lancaster book, you can study chapter no. 5, 6 and onwards. "pozar" book has got some very good examples too in chapter no 8. Pozar book can be starting point for microstrip filter design. If you can learn richards and kuroda identity, I think you can design any damn microstrip filter. Popular filters are stepped impedance filter, combline filter, hairpin filter, interdigital filter and many more depending upon applications.
Thanks