I suppose you are using the Microsoft MSVC compiler
Sorry, the libraries distributed by SpatiaLite have
been compiled using the free software compiler MinGW;
and this compiler (as any other standard GCC based
compiler) supports .a or .dll.a library files.
.lib is a 'not standard' format supported only by MSVC
uncontrolled rumors say that simply changing the suffix
is enough to transform a .a or .dll.a into a .lib ...
but I obviously will *not* assume any responsibility
about this ;-)
Anyway, the aim of SpatiaLite simply is supporting a
basic runtime env for Windows (tools and DLLs), not
redistributing many and many different libraries.
If you are a developer, it would probably better if
you'll build any required package from yourself starting
not at all a difficult task using the 'standard' MinGW
compiler; you simply have to follow the detailed how-to