Listmania!
Comparatively simple guides to microbiology & immunology
Complete Idiot's Guide to Microbiology (Complete Idiot's Guides (Lifestyle Paperback))Complete Idiot's Guide to Microbiology (Complete Idiot's Guides (Lifestyle Paperback)) by Ph. D. Byrd
Buy new: £8.40 / Used from: £11.19
Introduction to Microbiology (Eleventh Hour  Boston)Introduction to Microbiology (Eleventh Hour Boston) by Darralyn McCall
Buy new: £18.05 / Used from: £14.17
Very accessible.
How the Immune System Works (Blackwell's How It Works)How the Immune System Works (Blackwell's How It Works) by Lauren M. Sompayrac
Buy new: £15.03 / Used from: £12.99
The Flesh and Bones of Immunology (Flesh & Bones)The Flesh and Bones of Immunology (Flesh & Bones) by Matthew Helbert FRCP FRCPath PhD
Buy new: £14.82 / Used from: £13.97
More complementary to than identical to the Blackwell immunology book
Understanding Immunology (Cell and Molecular Biology in Action)Understanding Immunology (Cell and Molecular Biology in Action) by Dr Peter Wood
Buy new: £31.23 / Used from: £32.03
Not exactly simple but fairly straightforward, for this subject, with summary points at the end of each chapter.