A new Webelos leader, yay! Welcome to the club; we have the most fun position in all of Scouting! I may be biased of course.
There are a few things I suggest you read as soon as you can; fortunately much of it is online.
First of all, read the entire Webelos Handbook start to finish. Of all the books for you to be familiar with, that is the most important.
Then there is the Cub Scout Leader Book, which goes over all the basics of the Cub Scout program. The print copy can be found at your local Scout Shop or here https://www.scoutshop.org/cub-scout-leader-guide-646725.html and the digital copy can be procured here https://www.amazon.com/dp/B07HM7JVHT/ref=cm_sw_r_cp_ep_dp_aqrTBbCZM688E
The Webelos Den Leader Guide will have everything you need to plan and execute your weekly den meetings, and it is also full of useful information and aids for the program in its extensive appendices. Print copy available at your local Scout Shop or here https://www.scoutshop.org/webelos-den-leader-guide-646724.html or you can get a digital copy here https://www.amazon.com/dp/B07HMFF853/ref=cm_sw_r_cp_ep_dp_VprTBb72MZ4P8
These last ones will go pretty deep into all the rules and regulations, but they DO matter, so you'll want to be somewhat familiar with them at least as you begin, with the hope being you'll have a solid grasp of their content as soon as you can invest some time in reading them thoroughly. And fortunately, they are all available for free online. They are the Guide to Safe Scouting https://www.scouting.org/health-and-safety/gss/toc/ the Guide to Advancement https://filestore.scouting.org/filestore/pdf/33088.pdf and the Guide to Awards and Insignia https://www.scouting.org/resources/info-center/insignia-guide/
There is even a full webpage on the Scouting site dedicated to Den Meeting resources. Find it here: https://www.scouting.org/programs/cub-scouts/den-meeting-resources/
Obviously you don't have to read all of this TODAY, but whenever you have the time, try to work your way through them all. If you can be comfortably familiar with all of these resources, you will be miles ahead of many other leaders. Good luck to you!