Hello
louisd8 - To be honest which material to refer completely depends on the respective individual.
So for example, someone might have scored 700+ using Kaplan Books, however, you are facing some challenges and it can be a vice a versa.
Here are some of the reputed courses in the market, but before you go ahead and purchase these courses, I highly recommend you to take a trail. Purchase the course only if you like it.
SC: Book of Manhattan is Grear, though you will have to read it more than one time as not all the concepts will be clear in one reading. I also have heard rave reviews about E-GMAT specifically for SC, and do not hesitate to take a trial for this.
CR: Nothing Beats Power Score when it comes to CR and it is genuinely regarded as "The Bible" for CR
RC Veritas RC Strategy is good and you can club OG passages for practice along with this
Quant: Theory - Manhattan Guides and Tests -
GMAT Club Tests(The Best!)
Please note this is just personal opinion, and as I said what works for me may not work for you, so take this with a pinch of salt.
Hope this helps and wish you all the best