If you understand hebrew seforim – R’ Shimshon Pincus wrote a sefer called Sheearim Bitefila – it is only in Hebrew tho.
In english, there are the Praying with Fire books (kavana), R’ Shwob on Prayer and the Pathway to Prayer (translation).
There are lotsa Shiurim – check out TorahAnytime.com – on there I have recently seen a few by Chevy Garfinkel (one is called The Indestructible Soul and it was really good! but thats the only one I listened to so far.
All the books and sites everyone else said are also really good!
G’luck!