Those Marvel encyclopedias they publish. One like these where they just list a bunch of characters from A to Z with some info on their lore and first appearance.
Earth's Mightiest Hero's cartoon show was also a way I learned about other Marvel characters when I was younger.
I also wondered what the book was and I found it on Amazon (at least I think it is according to the pictures)
Marvel Encyclopedia, New Edition https://www.amazon.com/dp/1465478906/ref=cm_sw_r_cp_api_glt_fabc_ZSSWYBWA9F194494VXCS
Marvel Encyclopedia
Hard Cover coffee table book. I’ve got both the Marvel and DC versions.
Deal link: Amazon
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