What is the best book for ICAO English?
Best book for ICAO English is a real-life experience. ICAO English exam consists of non-standard phraseology assessment of an English language proficiency. Same like one can not learn to fly an aircraft by reading the books, also to learn an ICAO English, it needs some "hands-on" experience.
ICAO English exam also focuses on grammar structures, comprehension and pronunciation. You can brush up those language proficiency descriptors even by attending the basic language proficiency training course.
To improve your aviation-related vocabulary, a lot of different aviation-related sources can be used.