Back when 5e first came out, I made a one-page index of all the Monster Manual monsters, sorted by CR, with page numbers for each monster. Oddly enough, the Monster Manual didn’t have such an index!
Since I made it, I’ve found it insanely useful. I keep the PDF on my phone, and I have a copy taped to the inside back cover of my Monster Manual. I’ve used it before and during every D&D session I’ve run for the past 4 years.
Over the last four years, WOTC has released two major monster books: Volo’s Guide to Monsters and, as of a few days ago, Mordenkainen’s Tome of Foes. I’ve updated my PDF to include the hundreds of new monsters from these books. The PDF has ballooned up from 1 page… to 2 pages. Not a lot of edition bloat so far. In fact, there was some room left on the second page, so I included encounter-building guidelines distilled from Xanathar’s Guide to Everything. (My D&D group runs a bit hot so I have to throw more monsters at them than Xanathar suggests, but it’s a starting place.)
I hope you find this useful. Since it’s only 2 pages, you can still tape it to the left and right back cover pages of your Monster Manual.
Hey, this looks handy! I’ve also made something that could be useful to D&D players. It’s a handmade spellbook. A beautiful way to keep track of a characters spells.
You can read more about it here:
https://www.kickstarter.com/projects/rabbittownpress/spell-book-a-dungeons-and-dragons-spellcasting-not