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Supported File Formats
crnlib supports two compressed texture file formats. The first format (clustered .DDS) is simple to integrate into an existing project (no code changes are typically required), but it doesn't offer the highest quality/compression ratio that crnlib is capable of. Integrating the second, higher quality custom format (.CRN) requires a few typically straightforward engine modifications to integrate the .CRN->DXTn transcoder header file library into your tools/engine.
.DDS
crnlib can compress textures to standard DX9-style .DDS files using clustered DXTn compression, which is a subset of the approach used to create .CRN files. (For completeness, crnlib also supports vanilla, block by block DXTn compression too, but that's not very interesting.) Clustered DXTn compressed .DDS files are much more compressible than files created by other libraries/tools. Apart from increased compressibility, the .DDS files generated by this process are completely standard so they should be fairly easy to add to a project with little to no code changes.
To actually benefit from clustered DXTn .DDS files, your engine needs to further losslessly compress the .DDS data generated by crnlib using a lossless codec such as zlib, lzo, LZMA, LZHAM, etc. Most likely, your engine does this already. (If not, you definitely should because DXTn compressed textures generally contain a large amount of highly redundant data.)
Clustered .DDS files are intended to be the simplest/fastest way to integrate crnlib's tech into a project.
.CRN
The second, better, option is to compress your textures to .CRN files using crnlib. To read the resulting .CRN data, you must add the .CRN transcoder library (located in the included single file, stand-alone header file library inc/crn_decomp.h) into your application. .CRN files provide noticeably higher quality at the same effective bitrate compared to clustered DXTn compressed .DDS files. Also, .CRN files don't require further lossless compression because they're already highly compressed.
.CRN files are a bit more difficult/risky to integrate into a project, but the resulting compression ratio and quality is superior vs. clustered .DDS files.