It's actually an aspect of prepping that most never really consider. What to do during downtime.
For myself I've actually thought about it some.
Now sticking with D&D non-electronic only.
1. 3.5E PHB
2. 3.5E DMG
3. 3.5E MMI
From those you can build everything else.
4. 3.0E Forgotten Realms
5. 3.0E Epic Level Handbook
6. Spell Compendium
7. Magic Item Compendium
8. Fantasy Flight's Dawnforge book (the all 3 in one book) it's designed for D&D 3.0E
9. World of Greyhawk boxed set 1E
10. Dragonlance Campaign Setting 3.xE
With that you have the core rules which everything can be built off of, as well as Spell and Magic lists. You get four well rounded worlds to use.
A textbook on stats
A textbook on Combinatorics/Discrete math
A small book of tables with precomputed things like distribution of multiple dice rolls.
An Anthology of Myths and Legends
10 seams like a lot of D&D books... but here it goes
1. Pathfinder Core
2. Pathfinder Advanced Players Guide
3. 3.5 Monster Manual
4. Tome of Horrors Revised
5. Tome of Horrors 2
6. Tome of Horrors 3
7. Grimtooths Wurst Traps
8. Forgotton Realms 3.0 Campaign Book
9. A Pirates Guide to Freeport
10. Book of Fiends
1. BECM Compendium
2. 4e DMG
3. 2e Monstrous Compendium
4. 1e DMG
5. 4e Gamma World
6. Red Box D&D (for reminiscing of the World that Was)
7-10 round it out with modules or perhaps a campaign setting, although I have plenty of time to do my own
Ten's more than I ever need. But here's what I'd go with:
1. D&D Rules Cyclopedia
2. Classic D&D Game
3. Wrath of the Immortals
4. D&D Creature Catalog
5. Heroes of the Princess Ark
6. D&D Expert Set (with X1 Isle of Dread)
7. PC1 Tall Tales of the Wee Folk
8. PC2 Top Ballista
9. PC3 The Sea People
10. Gaz10 Orcs of Thar