As the title says this is going to be a resource guide to the Forgotten Realms and if you want to start a D&D campaign.
First of all you need three standard items.
The Player's Handbook Dungeon Master's Guide Monster Manual
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Other additions thought not entirely necessary are:
1. Arms and Equipment Guide
2. Fiend Folio
3. Book of Vile Darkness
4. Savage Species
5. Epic Level Handbook
6. Monster Manual II
7. Psionics Handbook
8. Manual of the Planes
9. Deities and Demigods
Info on all of these can be found
here. and even more books
here. . Though personally the second link is not all that important.
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Specifically for the Forgotten Realms Campaign Setting you can look
here.
Other books which you may want/need are:
1. Races of Faerūn
2. Faiths and Pantheons
3. Magic of Faerūn
4. Monsters of Faerūn
Books that are specific to certain specific regions or certain classes/alignments you may want to embark on in your campaign are:
1. Unapproachable East
2. Silver Marches
3. Lost Empires of Faerūn
4. Lords of Darkness
5. Shining South
6. Underdark
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I know I am missing some books and texts. If you feel any need to be mentioned please pm me. Thank you.