I recently found the Encode Thoughts cantrip in the Guildmaster's Guide to Ravnica, which has left me wondering if there are any other spells that I am missing.
What official sources contain spells (as of 29/06/19) other than the Player's Handbook, Elemental Evil Player's Companion, Sword Coast Adventurer's Guide, and Xanathar's Guide to Everything?
Best Answer
To find out if a source has any spells, you can go to D&D Beyond's spell search and filter by sources. This will work even if you don't have the books purchased on D&D Beyond. The sources at the time of my most recent edit that have spells are:
(for some reason the Dungeon Master's Guide and Monster Manual are listed as having all of the spells in the Player's Handbook, but I can confirm that they are not in there.)
(this book reprints wall of water as it appears as a racial spell for Tritons, but for some reason D&D Beyond does not show that fact )