We do not make any particular recommendation - you are free to choose and use them free of charge. They do, however, fall roughly into the following age categories:-
(NB: I've added some notes, ' ...', to the titles.)
- Age 6-7: Chess: The First Year of Study
The suggested catalog reference (is this the correct term?) explains,
'The textbook “Chess: The First Year of Study” is an introductory course of chess for kindergarteners and was created as a project of Chess in Schools.'
- Age 7-9: Elementary Level Chess Class Book (Kulac)
Starts with an introduction to FIDE.
- Age 9+: Welcome to Planet Chess! The introduction starts, 'This course teaches you all you need to know about how the pieces move and the rules of the game, so that you can get full enjoyment from playing chess.'
In the 'Age 6-7' entry, I noticed a number of technical errors probably due to translation. Page 18 says, 'Complete the sentence. Chess is a game for two ????????', i.e. eight boxes for the word 'players'; the two other statements have the same problem. In the associated workbook, page 7 says, 'Find the 8 ranks', when it means 'files'. There are undoubtedly other errors. Both books need better proofreading.
The 'Age 9+' entry is the simplest of the three. There is almost nothing beyond an explanation of the rules of the game. Is this the first part of a work in progress?
The 'Age 7-9' entry is a full introduction to chess for children. It was the first curriculum offered by FIDE, is more comprehensive than the other two, and appears technically robust on first reading. If I had to choose one of the three entries for further work, I would pick this one.