Latex Spanning a table horizontally over pages

Question

I have a very wide table, which I like it to span horizontally through pages.

    \documentclass[runnngheads]{book}
    \usepackage{longtable}
    \begin{document}
    \begin{longtable}{*{25}c} 
        \hline 
        $a_1$ & $a_2$ & $a_3$ & $b_1$ & $b_2$ & $c_1$ & $c_2$ & $c_3$ & $d_1$ & $d_2$ & $i_1$ & $i_2$ & $e_1$ & $e_2$ & $h_1$ & $h_2$ &test&test 2&test 3 &test 4 & test 5 & test 6 & test 7 & test 8 & test 9\\ 
        \hline 
    \end{longtable}
    \end{document}

Any idea how to do it? There are plenty of examples for vertical spanning but I could not find any for my case. And I do not want to use landscape mode as well.