best books of microwave engg.
Your question seems to be very generic - microwave engineering is as broad of a topic as engineering itself and needs to be refined. It largely depends on what you need it for. If you re dealing with passives - then - what passives - transission lines or lumped? If this is lumped - Collin and Pozar will lead you to nowhere - but on transmission lines they will probably serve best - Pozar is more akin to engineering while Collin texbook is more theoretical and in my opinion a bit oudated. Again, if you have knowledge on electromagnetics both texts will do. On passives - you'd better look elsewhere.
Now - what if you wanted to design active microwave circuits- after all microwave engineering is not all about waveguides, isn't it.
Then you will rarely find something better than Pavio, Vendelin and Rohde on the subject (John Wiley). It is wonderful text and will give you may "clues" to good design practices - unlike the book of Newkirk and Rohde that is terrible to say the least.
Therefore, make attempt to define what you need the book for - what your background is and you will find mostly anything inhere.