A cross slab is a carved slab of local stone, usually a grave memorial, which bears a representation of the Christian cross. A cross is a carved representation - usually ring-headed.
This bald description does no justice to the amazing range of styles and artistic decoration to be found. Around 200 such crosses are known falling into three main categories:
The simple compass arc decorated slabs dating from around 650 to 800AD,
the later and much larger Celtic ring headed crosses of
the 9th Century showing more Scottish than Irish influence
as in the Lonan Cross shown here.
The final flowering of the 10th and early 11th Century Norse crosses (as for example Kirk Michael) with their interlaced chains and zoomorphic figures illustrating a mixture of Norse folklore and Christian subjects.
Mention of the Runic inscriptions had been made by Bishop
Wilson. The first published text on the Crosses was that by
Kinnebrook in 1841 (though he merely gave 24 sketches and
measurements, the first serious study was that of Cumming
in 1857. Kermode was
probably provoked in producing his first edition of Manx
Crosses by a patronising reply by the famous Icelandic scholar
Prof Vigfusson who on the basis of a quick visit to the Island had
made a series of somewhat hasty judgements which Kermode had queried.
Kermode's initial interest seems to have been in the inscriptions as
seen in the first two editions.
The third edition of Manx Crosses expands on the art and explains much of the Norse background to the figures. It is now a highly sought after collector's item.
A 1994 reprint has the great advantage of additional material in the appendices as well as David Wilson's excellent introduction placing Kermode's work into the context of later scholarship.
A.M.Cubbon's Manx Museum guide is probably the most accessible. Maureen Costain Richards' book is highly colourful and enjoyable - it is an artists response to the images on the crosses - these images are brought to life by the use of colour.
Alfred Marshall Cubbon The art of the Manx Crosses Douglas: Manx Museum & National Trust 1971 - 3rd ed 1983
William Kinnebrook, Etchings of the runic monuments in the Isle
of Man London: Longmans 1841
J.G.Cumming The Runic and other Monumental Remains of the Isle of Man LondonLondon: Bell & Daldy 1857
Phillip Moore Callow Kermode, 1855-1932 Catalogue of the Manx Crosses... Ramsey: J.Craine 1887
P. M.C. Kermode, Catalogue of the Manx Crosses...2nd ed Isle of Man: C.B.Hayes Ramsey Courier Office and London: Williams and Norgate 
P. M.C. Kermode, Manx Crosses London: Bemrose & Sons 1907
P M.C Kermode + David M Wilson Manx Crosses Balgavies: Pinkfoot Press 1994 (ISBN 1-874012-07-5 pbk 1-847012-08-3)
Mareen Costain Richards Manx Crosses - Illuminated Port St. Mary: Croshag 1988 (ISBN hbk 0-9513416-0-X pbk 0-9513416-1-8)