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Well that kind of depends on what it is for and at what level. Do you want something that is quit complete for someone who is to be a filter engineer lets say
or do you need to design a filter or is it a school type thing.
Mathaii young jones listed above is by far the most complete and if you can understand the book it will tell you how to design just about anything.
There is a book from Randall Rhea that is a little simplier
I think it is called microwave filters and simulation or something like that
The microstrip filters book above is also quit good
A combination of these is probably the best choice