LibXL - A proprietary Excel writing library written in C++. Supports reading, writing and editing Excel files. Basic licenses for iOS are $199, licenses including source code are $2499. I didn't end up using libXL, but I might have if one of the free, open source alternatives didn't work.
JXLS An open source (BSD licence) Objective-C++ wrapper for the open source (BSD licence) xlsLib .xls writing library for iOS. Actively maintained. CocoaPod available. Seems to support xls files only. There's a counterpart library for reading xls files too.
libxlsxwriter An open source (BSD licence) Excel writing library written in C. Libraries by the same author exist for Perl, Lua and Python. Supports xlsx files. No CocoaPod or XCode project available, but build process is simple (without autotools).
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