I (along with many others I suspect) am interested in getting a Commercial Drone "Remote Pilot Certificate"
I'm finding a paucity of study guides.
So far I've found this.
https://www.buildadronebiz.com/product/test-study-guide/

I'm wondering if people know of others and whether it helped them pass the test

Adding this. There's a few critical reviews though.
https://www.amazon.com/Remote-Pilot-Test-Prep-UAS/dp/1619544687/

Try this - go to phantompilots.com and under the Rules and Regulations sub-forum read the comments on the online study guides.

I see references to this
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/remote_pilot_study_guide.pdf
Perhaps studying this is sufficient?

I have passed my 107 and now have my UAV Pilot's license from the FAA.
I used the 'Study Buddy' app which is on the App Store and cost $9.99.
It is for private pilots who are studying for their license. It quizzes you on
the questions you will need to pass the 107.....as well as quite a few that you
won't . Fortunately, you can have it only ask you questions on the areas you
select. Just deselect areas which obviously won't be on the 107 test...
things like 'Powerplant' (because you don't need to know about plane engines)
or 'Landing Gear'. Keep everything else selected, the weather and weather theory,
the airport operations, sectional charts and so on. What is nice about the app, is
it will not only tell you if you got the question right, and if not, what the correct
answer is, but it explains WHY as well.......very valuable when you don't have a clue
on any of this stuff like me. I studied for about 3 weeks until I could basically get
100% on the practice tests it gave me. Even then, I missed more questions than
I thought I would on the 107 test.....it can be pretty tough. Make sure you put the
study time in, it's not going to be something you can study for a couple hours and pass
unless you get lucky (each test is different as you get questions randomly selected from
several hundred possible questions.....so some tests are easier than others.)

I started studying the dizzying array of materials referenced by the FAA, the other day i broke down and ordered the study book craig linked.

never thought about an app, but im probably going to download that too, good find gabe!

There's tons of very good info out there to help you pass-- for free.

Just about everything you need to know with plenty of links to supplementary material and instructional videos:
Sarah Nilsson - UAG Test Prep - 1 (http://www.sarahnilsson.org/aviation/uag-test-prep-1/)

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This site is targeted toward pilots with existing private licenses who only need to take this test and submit paperwork, but anyone can setup an account and navigate through the course. Very well presented with test questions along the way.

Part 107 for small UAS Course
https://www.faasafety.gov/gslac/ALC/course_content.aspx?cID=451&sID=733&crID=1436761

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Here's the actual material that you'll have available when taking the test. Many of the questions reference figures in this document:
FAA-CT-8080-2G, Airman Knowledge Testing Supplement for Sport Pilot, Recreational Pilot, and Private Pilot
https://www.faa.gov/training_testing/testing/test_questions/media/sport_rec_private_akts.pdf

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Some other sites that would be well worth your time to review:

UAG Sample Exam - Federal Aviation Administration
https://www.faa.gov/training_testing/testing/test_questions/media/uag_sample_exam.pdf
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Remote Pilot – Small Unmanned Aircraft Systems Study Guide
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/remote_pilot_study_guide.pdf
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FAA Aeronautical Chart User's Guide
https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/aero_guide/
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The BEST source IMO for understanding weather codes:
Aircrew Quick Reference to the METAR and TAF Codes
http://chesapeakesportpilot.com/wp-content/uploads/2015/03/military_wx_codes.pdf
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I (along with many others I suspect) am interested in getting a Commercial Drone "Remote Pilot Certificate"
I'm finding a paucity of study guides.
So far I've found this.
https://www.buildadronebiz.com/product/test-study-guide/

I'm wondering if people know of others and whether it helped them pass the test