Though I haven't use Sheppard Air test prep, I just got through this test last week and can talk to a slightly different approach that I tried.
The sources I used were Rod Machado's IFR handbook app that I downloaded on my ipad and I also used the dauntless IFR app on my ipad as well. It was a bit of an experiment to see if I could only use the ipad for prep.
They were the only two apps I used and spent time over the holidays to get the time in.
I highly recommend the dauntless app. I used a feature on their app that ranks questions depending on whether you answered the question correctly or not. The neat thing about the feature is that you can choose to no longer show questions in the "5" bucket - means I had answered the particular question correctly 2 times at some point. The questions did at some point get monotonous and repetitive. But then again that was when I felt I was ready. BTW, you do want to save your particular session as the app will often reset when you put the ipad down for some time midstream during a set of practice questions. Dauntless wasn't a perfect match to the actual exam questions but the concepts were all there so no surprises.
As for Rods book. I thought his presentation of the material was decent. But I honestly started to get annoyed by the inserted humor as it often was really forced. I ended up just skipping passed all of these paragraphs whenever he departed from the core material. It felt like wasted time to read through it. Also as a heads up I noted several mistakes in the book. Not really material and were easy enough to spot. I just assumed that there were updates in the version that I was using on my ipad that hadn't been thoroughly proofed.
Using these two apps, I felt more than ready. Also I thought the price of the dauntless app in particular was better than other options out there. Plus I really valued the added bonus of having these materials with me on my ipad whenever I had a dozen minutes or so of spare time. Hard to carve the time out with two kids and family over for the holidays so the convenience factor of using my ipad was significant.
No relation to either of these companies.
Good luck!