My copies of the 1E Core Three have by far seen the most use over my 40+ year RPGing career; 1E PHB most of all. My 3.5E Core Three have seen the second-most amount of usage, though by some great margin, with the 3.5E PHB taking the most abuse of those. I'm in and out of others for reference, from time to time when worldbuilding and such. I daresay that my
3.5 SRD Revised PDF Bundle has been utilized even more than my 3.5 Core Three books. It's much more useful to me and is always at hand. I can copy and paste stuff from it in an instant. I can do a quick search and find almost anything in three clicks or less because of the bookmarks and linking table of contents structure. It's probably saved my 3.5 Core Three from falling apart, too.
When I first released it, it was just a couple PDFs covering the full
SRD: Monster as well as Basics, Spells, and Magic items. These were heavily hyperlinked and bookmarked for easy use. This originally sold for $5 and quickly became a hot seller. Then, individual PDFs were added for the spells of each spellcasting class. A separate PDF for the Psionics rules was added, then one for the additional Divine rules and one for Epic rules. Pentultimately, a Creature Stat Blocks PDF was constructed taking all of the
SRD monsters and laying them out in standard stat block format. Finally, an Index page was made that tied all of the other 16 PDFs together and each PDF in the bundle was linked back to the Index page. It's all amazingly fast for anyone needing a quick rules clarification or to check monster stats or spell details. The price has gone up over the years as more and more was added to the original product but it is on sale for the holiday for the original $5 price so grab one
here.