His book prayer is a good one to start with, I’m going through it and he can get fairly deep but in a good way. The only other stuff of his I have read was an afterword in the book meditations on the taro: a journey into Christian hemetisim by anonymos (pubulished after the unknown authers death) which is a very deep book on the mysteries of the christan faith (it does conatin some errors but also many many good teaching on mystisim if you are knowledgable in the catholic faith then it could be “safe” to read and is endorsed by a number of trappist abbots and priests). But balthasar’s book prayer teaches contemplative prayer and I highly endorse it. He does get very tecnical in his language and can be difficult to understand but if you press on it can be very rewarding. Do not be intimidated just becuse his work can be hard to understand if you keep at it and think it through you will achive what hans set out to do get people thinking and growing in their faith.
