Sorry to hear about your brother's passing.  He had a nice lot of books.  There are probably a lot of fond memories from them.

The value is based upon the amount of work one would be willing to do to sell them.  Selling them as a lot is easier, but the resulting return is usually quite a bit less than selling them individually.  eBay would be your best option for selling them, though keep in mind that eBay takes about 15% in fees from the final sale.

Here are some estimated valuation ranges for the items.

AD&D & D&D Rulebooks:

Dungeons and Dragons; Rules for Fantastic, Medieval Role Playing Adventure Game Campaigns, Third printing May 1978, cover heavily sun bleached and worn, some staining, pages in good repair. $30 – $40

Fiend Folio 1981, 0-935696-21-0, near mint condition $45 – $50

Dungeon Master’s Guide Revised 1979 0-935696-02-4, well worn cover, pages in good shape $25 – $30

Player’s Handbook 6th printing 1980 0-935696-01-6, cover highly worn, some outer-binding material loss with decades-old tape repair, pages in good shape $15 – $20

Monster Manual 4th edition August 1979 0-935696-00-8, cover highly worn, binding partially disconnected in top half, small amounts of staining on pages $15 – $20

Modules, & Accessories:

The World of Greyhawk (presumably complete) 1980 0-935696-23-7, map in good shape with thumbtack holes in perimeters and small tape marks, binding folder has some corner wear but intact $45 – $60

Rogues Gallery 1980 0-935696-18-0, gently worn, staples have torn out of front cover, otherwise intact $12 – $15

Dungeon Module D1-2 Descent into the Depths of the Earth 1978, 1981 0-935696-08-3, good condition, only small amounts of edge wear $25 – $35

Dungeon Module G1 Steading of the Hill Giant Chief 1978 0-935696-60-1, good condition, only small amounts of edge wear $20 – $30

Dungeon Module G2 the Glacial Rift of the Frost Giant Jarl 1978, 0-935696-09-1, good condition $20 – $30

Dungeon Module A1 Slave Pits of the Undercity 0-935696-25-3, good condition $20 – $25

Dungeon Master screen & Advanced Dungeon Master screen, 1979, good condition, small amounts of staining $15 – $25

Non TSR items:

City State of the World Emperor SHOPS by Judges Guild Incorporated 1980, good repair, pencil writing throughout as banker notes & City State of the World Emperor CITY by Judges Guild Incorporated 1980, pencil writing throughout as tax/toll collector notes $20 – $30

Best of WHITE DWARF Scenarios, 1980, cover heavily worn with some binding separation, pages in good repair $10 – $15

Total Estimated Valuation (Individually): $315 – $425
Total Estimated Value (Lot Price): $225 – $300