https://www.humblebundle.com/b...

These are electronic books (PDFs and other formats).

Humblebundle sell in tiers - you get the tier that you paid for and all previous tiers. So, for $18, you get everything listed below (and many non-gardening books as well)

$1
Grow a Living Wall
Container Gardening Complete
101 Organic Gardening Hacks
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$5
Countertop Gardens
Straw Bale Solutions
Square Foot Gardening: Growing Perfect Vegetables
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$10
Miniature Gardens
Square Foot Gardening High-Value Veggies
Raised Bed Revolution
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$18
Straw Bale Gardens Complete, Updated Edition
Step-by-Step Projects for Self-Sufficiency
Square Foot Gardening Answer Book
No-Waste Organic Gardening

Expires on the 19th of January.