There is one supposed to be published in 2012, but it is now long overdue. The best alternatives that I have been able to find information on were published more than 50 years ago and simply are not available in the market.
The book entitled ''Lezards, crocodiles et tortues d'Afrique occidentale et du Sahara'', co-authored by Jean-Francois Trape, Sebastien Trape et Laurent Chirio and published in 2012 is by far the best reference for West African non-ophidian reptiles. It is available at IRD Editions (maybe you can get it faster through Laurent Chirio who is on Research Gate). It is in French but there are amazing photographs and excellent distribution maps. For the snakes of West Africa you can use the book on snakes by Chippaux (also in French). East of that area, you will have to use several guides, and several countries are unfortunately not adequately covered. But for a number of them several groups are well covered in various papers. The list of papers and books to use depends on what you need in particular.
Merci Olivier Pauwels, thanks Szabolcs Safian, grazie Leonardo Latella and merci Xavier Glaudas -- all of your naswers were helpful! There is an English language book in pipeline, but is is now over a year late so I'll see if I can access the French books that have been suggested.
You may find the following work useful, see also the review of Olivier Pauwel's RG profile: Atlas des Reptiles du Cameroun, by L. Chirio & M. LeBreton. Muséum National d’HistoireNaturelle & IRD Editions, Paris. 2007. Price 58. ISBN 978 2 85653 603 2.