Yes it is expensive and Hein is not the most famous bass player there is. But this is the best double bass method there is to REALLY improve your playing (and I tried them all: Sher, Carter, Brown, Reid, Simandl). I own both books (lucky enough I live in Holland and I could buy them cheap from my bass luthier).
Book I is fantastic but personally I study a lot in book 2 and especially the numbers under each note mentioning which finger to use are very helpfull to really improve your intonation and fingering. I have been using this book for 4 months now and several hick ups from the past in my playing have already disappeared. If you want flashy tips on solos from a famous bass player or transcriptions from Chambers or La Farro don't buy this book. If you want to become one with your instrument you can use this book for the rest of your life.