I usually go with lonely planet and they do have a gambia and senegal book but it annoys me how there are never pictures ( apart from the top 10 at the start).
for gambia we are actually doing the "package holiday" thing rather then independent travel. there are day trips to senegal and so for this reason i find it very useful to have a book with both it. the history section at the start is really good as are all the pictures to give you a sense of what to see. unless you are going on independent travel then i'd definitely recommend this book- even then you could still get by with this just fine.
never bought an insight travel book before but definitely would again