These two books are part of an unknown series. Their spines are the same as those seen in Format 1 books from Series 157 Representative Poets Series (http://henryaltemus.com/series/series157.htm).

These two books: Idylls of the King and Lady of the Lake were published before 1900 (Publisher is Henry Altemus not Henry Altemus Company). Both of the titles are included in the Representative Poets Series list. Thus it is most likely that this cover format is part of Series 157.

Unfortunately a similar spine is not enough. Until a book is found with a labeled box or a dust jacket, placement into a series will be an educated guess.

Here are other series possibilities:
#47 Beauxarts Series
#83 Fairmont Series
#119 Laurel Series
#141 Petit Trianon
#155 Representative English Poets
#162 Sanspareil Series
#179 Vademecum Series
#181 Valenciennes Series