Different parts of the repository are licensed differently.
The ebook itself (contained in the doc folder) is Copyright (c) 2018 Daniels Umanovskis under the Creative Commons CC-BY-SA license - human-readable summary.
The source code under src, except as noted below, is Copyright (c) 2018 Daniels Umanovskis under the MIT license.
The modified U-Boot source code under src/common_uboot is Copyright (c) Wolfgang Denk and DENX Software Engineering under the GPL-2.0. See U-Boot license statement.