So there are probably not too many texts that describe Phase I and IIa trials. In fact, different companies even classify IIa/IIb differently. It's probably easier to google this and find some professor's powerpoints on the matter.
Here's the quick low-down. Phase I trials are - in concept - trials to test safety and toxicity alone (they rarely factor in efficacy tests of the intervention in question). Usually healthy volunteers (eg. do not have the disease in question) are recruited and undergo pretty intensive testing (often stay at a medical facility for more than 24 hours). During this phase as well, most trial designers include a pharmacokinetic/pharmacodynamic (PK/PD) study to understand what's going on with the drug inside the body, since this is the first trials in man. Some companies call these studies IIa. Other companies call IIa "Proof of concept" studies. That is, testing the drug early on to see both if it works and if it's safe - a mini-Phase III trial, if you will. Phase II are smallish trials that try to test both efficacy and safety in a low-cost, minimalistic manner. The trials are, however, very highly controlled and the most scientifically rigorous (in broad terms) than the others. Clinically, this means often drug interactions are excluded from study hypotheses (eg. recruit sick asthma patients for our experimental asthma drug, but they can't be too sick to need other medications), or excluding patients with a number of comorbidities that might confound outcomes.
Beyond Phase I and II, here are two great books that my boss' boss (VP of Clinical Research at top 3 pharma) recommended to me.
Designing Clinical Research - Hulley, et al
This is a good primer book for all of the concepts surrounding clinical trials - somewhat technical
Clinical Epidemiology: The Architecture of Clinical Research by Alvan R. Feinstein
This is a highly technical book, but an outstanding text on the mechanics of clinical research. My VP mentor said his own mentor (a world-reknowned clinical researcher) said all you need is this book to succeed in clinical research. Came highly recommended.
tbo
