There are indeed many ways to study for the FAA Remote Pilot Certificate. Several excellent online sites offer instruction, sample tests, and helpful information. The Recurrent (renewal) test is 40 multiple choice questions, of which airspace knowledge is an important part. Weather, Loading and Performance questions have been dropped from the Recurrent test.
Should you wish to self-study, ASA, Aviation Supplies & Academics, is a premier supplier of prep booklets for anything connected with aviation, be it manned or whatever. They have excellent co-ordination with FAA requirements.
The ASA current training manual for the Remote Pilot tests is "2019 Remote Pilot Test Prep", and covers both the initial and recurrent tests, and includes the FAA Knowledge Supplement. Although this can be purchased direct, it is actually cheaper from 3rd party suppliers. Here is an Amazon link: Check it out