Books comprising a series are usually grouped together. The non-series books I arrange according to subject matter: chem, non-chem but science-related, everything else.

Sometimes, under "everything else" I group books by the same author, but I don't arrange them alphabetically. More often than not, they're arranged by cover-quality: hardbound, trade-book, paperback.

THEN they're packed in plastic crates and stacked in a corner of the house. We have no more shelf space, alas!