I’ve heard good things about My Prayer by R’ Nissan Mindel, which has recently been reprinted, but I haven’t read it myself.
Here’s one quote from a chassidic sefer about kavanah:
“You must really force yourself to concentrate on your prayers. I disagree with the people who say one should not try to force it. It is very hard to pray, and people are usually not able to pray more than a portion of the prayers. But even if you sometimes cannot pray at all, the effort you put into forcing yourself to pray is also very precious to God, even if you don’t actually succeed in praying as you should. These efforts are accounted as sacrifices, and this is the meaning of the verse: `But for Your sake we are killed all the day; we are accounted as sheep for the slaughter’ (Psalms 44:23). This refers to the effort we put into our prayers even if we find it impossible to pray. This is a general principle in serving God. Even if we do not manage to serve Him in a way that is fitting, the effort we put in is still very precious, and in the worlds above it is accounted as a sacrifice” (Sichos HaRan 12).