I second Dauntless but by it self it is most likely not enough. I find the ASA test prep (it has glider specific questions and answers in their curriculum ) works real well between the two

I agree. It would be a false and risky approach to get a fragmented knowledge only for the sake of the written exam and to not learn all the basics in a systematic way, area by area, from a good handbook. A software like Dauntless prepares you specifically for the test, but luckily is also useful (and entertaining and at times challenging) as you progress and grow in wisdom and gain experience through actual flying.