There are lots of old cookbooks that have been digitized by various people (libraries, Project Gutenberg, Google...) and are free to read and download. The problem is getting to know about their existence. I'll try to post the ones I saved. If you know books that are missing in this list then please reply with the links, so I'll edit this post and add them, thanks.
Acton Eliza - Modern Cookery for Private Families
1847
https://archive.org/details/moderncookeryfo01actogoog/page/n8
Acton Eliza - The English Bread Book for Domestic Use
1857
https://archive.org/details/englishbreadboo00actogoog
Anonimo - Il confetturiere piemontese che insegna la maniera di confettare frutti in diverse maniere
1790
https://archive.org/details/b28766672/page/n4
Beeton Isabella - The Book of Household Management
1861
http://www.gutenberg.org/ebooks/10136
Bradley Richard - The Country Housewife and Lady's Director
1728
https://archive.org/details/countryhousewife00brad
Brown Susan Anna - The Book of Forty Puddings
1882
https://babel.hathitrust.org/cgi/pt?id=hvd.hn6gyz&view=1up&seq=1
Careme Marie-Antoine - Le Patissier Royal Parisien
1815
https://archive.org/details/b21504672_0002/
Careme Marie-Antoine - Le Patissier Pittoresque
1815
https://archive.org/details/b21525419/
Careme Marie-Antoine - Le Maitre d'Hotel Francais
1822
https://archive.org/details/b2152600x_0001
https://archive.org/details/b2152600x_0002
Careme Marie-Antoine - Le Cuisinier Parisien
1828
https://archive.org/details/b29300125
Careme Marie-Antoine - L'Art de la Cuisine Francaise au XIX Siecle
1833
https://archive.org/details/b21525687_0003
https://archive.org/details/b21525687_0004
Carey Henry - A Learned Dissertation on Dumpling
1727
http://www.gutenberg.org/ebooks/28105
Colmenero de Ledesma Antonio - Chocolate An Indian Drinke
1652
https://www.gutenberg.org/ebooks/21271
De La Varenne Francois Pierre - Le Cuisinier Francois
1651
https://archive.org/details/bub_gb_YpIorAokVqUC
De Salis Harriet Ann - Puddings and Pastry a la Mode
1889
https://archive.org/details/puddingspastry00desa
De Voe Thomas Farrington - The Market Assistant
1867
https://books.google.com/books/about/The_Market_Assistant.html?id=2z4EAAAAYAAJ
Dumas Alexandre - Le grand dictionnaire de cuisine
1873
https://archive.org/details/legranddictionn00dumagoog
Escoffier Auguste - Le Guide Culinaire
1902
https://archive.org/details/b21525730
Escoffier Auguste - A guide to modern cookery
1902
https://archive.org/details/cu31924000610117
Fisk + Ellenberger - An ice cream laboratory guide
1917
https://archive.org/details/icecreamlaborato00fisk
Gill Thompson - The complete practical pastry cook
1889
https://archive.org/details/completepractica00gill/page/n4
Glasse Hannah - The Art of Cookery Made Plain and Easy
1796
https://archive.org/details/TheArtOfCookery
Gouffe Jules - Le livre de cuisine
1807
https://archive.org/details/b21525778
Griswold Ruth Mary - Experimental Study of Foods
1962
https://catalog.hathitrust.org/Record/001043252
Hall Mary Elizabeth - Candy-Making Revolutionized Confectionery from Vegetables
1912
https://www.gutenberg.org/ebooks/33974
Kidder Edward - Receipts of Pastry & Cookery For the Use of His Scholars
1720
https://www.loc.gov/item/44028152/
Lamb Patrick - Royal cookery The complete court-cook
1710
https://www.loc.gov/item/44025907/
Lowe Belle - Experimental Cookery From the Chemical and Physical Standpoint
1932
https://archive.org/details/experimentalcook00lowerich
Maestro Martino da Como - The art of cooking
1480
https://www.academia.edu/15168285/Maestro_Martino_of_Como_The_art_of_cooking_-_the_first_modern_cookery_book
Marinetti Filippo Tommaso - La cucina futurista
1932
https://www.mori.bz.it/gastronomia/Marinetti - Cucina futurista.pdf
Murrey Thomas - Puddings and Dainty Desserts
1886
https://archive.org/details/puddingsdaintyde00murr_0
Nostradamus - Traite des Fardements et Confitures
1556
https://www.oldcook.com/doc/livre_nostradamus_confitures.pdf
Nott John - The Cook's and Confectioner's Dictionary
1723
https://archive.org/details/cooksandconfect00nottgoog
Nutt Fredrick - The Complete Confectioner
1819
https://archive.org/details/completeconfecti07nutt
Parloa Maria - Chocolate and Cocoa Recipes
1780
https://archive.org/details/chocolaterecipes_1403_librivox
Pease Martin - Candy Making Secrets
1908
https://archive.org/details/candymakingsecre00peas/page/n2
Rabisha William - The Whole Body of Cookery Dissected
1673
https://www.loc.gov/item/44028918/
Rundell Maria Eliza - A New System of Domestic Cookery
1806
https://archive.org/details/newsystemofdomes01rund
Scappi Bartolomeo - Opera
1570
https://archive.org/details/operavenetiascap00scap
Sherwood Snyder - The Art of Candy Making Fully Explained
1915
https://archive.org/details/TheArtOfCandyMakingFullyExplained
Simmons Amelia - American Cookery
1796
https://www.gutenberg.org/ebooks/12815
Skuse E - Skuse's Complete Confectioner
1894
https://archive.org/details/b28048465
Taillevent - Le Viandier
1486
https://www.gutenberg.org/ebooks/26567
Terhune Herrick Christine - Candy Making in the Home
1914
https://archive.org/details/candymakinginho00herrgoog/page/n8
Walsh J H - The English Cookery Book
1810
https://archive.org/details/englishcookeryb00bookgoog
Teo
